Your friend subscribes to the ideal of 'sola scriptura', that is -Scripture Only theology. Ironically, sola scriptura can be debunked using nothing but Scripture. So let's start with that:
Scripture Alone Disproves "Scripture Alone"
Gen. to Rev. - Scripture never says that Scripture is the sole infallible authority for God's Word. Scripture also mandates the use of tradition. This fact alone disproves sola Scriptura.
Matt. 28:19; Mark 16:15 - those that preached the Gospel to all creation but did not write the Gospel were not less obedient to Jesus, or their teachings less important.
Matt. 28:20 - "observe ALL I have commanded," but, as we see in John 20:30; 21:25, not ALL Jesus taught is in Scripture. So there must be things outside of Scripture that we must observe. This disproves "Bible alone" theology.
Mark 16:15 - Jesus commands the apostles to "preach," not write, and only three apostles wrote. The others who did not write were not less faithful to Jesus, because Jesus gave them no directive to write. There is no evidence in the Bible or elsewhere that Jesus intended the Bible to be sole authority of the Christian faith.
Luke 1:1-4 - Luke acknowledges that the faithful have already received the teachings of Christ, and is writing his Gospel only so that they "realize the certainty of the teachings you have received." Luke writes to verify the oral tradition they already received.
John 20:30; 21:25 - Jesus did many other things not written in the Scriptures. These have been preserved through the oral apostolic tradition and they are equally a part of the Deposit of Faith.
Acts 8:30-31; Heb. 5:12 - these verses show that we need help in interpreting the Scriptures. We cannot interpret them infallibly on our own. We need divinely appointed leadership within the Church to teach us.
Acts 15:1-14 – Peter resolves the Church’s first doctrinal issue regarding circumcision without referring to Scriptures.
Acts 17:28 – Paul quotes the writings of the pagan poets when he taught at the Aeropagus. Thus, Paul appeals to sources outside of Scripture to teach about God.
1 Cor. 5:9-11 - this verse shows that a prior letter written to Corinth is equally authoritative but not part of the New Testament canon. Paul is again appealing to a source outside of Scripture to teach the Corinthians. This disproves Scripture alone.
1 Cor. 11:2 - Paul commends the faithful to obey apostolic tradition, and not Scripture alone.
Phil. 4:9 - Paul says that what you have learned and received and heard and seen in me, do. There is nothing ever about obeying Scripture alone.
Col. 4:16 - this verse shows that a prior letter written to Laodicea is equally authoritative but not part of the New Testament canon. Paul once again appeals to a source outside of the Bible to teach about the Word of God.
1 Thess. 2:13 – Paul says, “when you received the word of God, which you heard from us..” How can the Bible be teaching first century Christians that only the Bible is their infallible source of teaching if, at the same time, oral revelation was being given to them as well? Protestants can’t claim that there is one authority (Bible) while allowing two sources of authority (Bible and oral revelation).
1 Thess. 3:10 - Paul wants to see the Thessalonians face to face and supply what is lacking. His letter is not enough.
2 Thess. 2:14 - Paul says that God has called us "through our Gospel." What is the fullness of the Gospel?
2 Thess. 2:15 - the fullness of the Gospel is the apostolic tradition which includes either teaching by word of mouth or by letter. Scripture does not say "letter alone." The Catholic Church has the fullness of the Christian faith through its rich traditions of Scripture, oral tradition and teaching authority (or Magisterium).
2 Thess 3:6 - Paul instructs us to obey apostolic tradition. There is no instruction in the Scriptures about obeying the Bible alone (the word "Bible" is not even in the Bible).
1 Tim. 3:14-15 - Paul prefers to speak and not write, and is writing only in the event that he is delayed and cannot be with Timothy.
2 Tim. 2:2 - Paul says apostolic tradition is passed on to future generations, but he says nothing about all apostolic traditions being eventually committed to the Bible.
2 Tim. 3:14 - continue in what you have learned and believed knowing from whom you learned it. Again, this refers to tradition which is found outside of the Bible.
James 4:5 - James even appeals to Scripture outside of the Old Testament canon ("He yearns jealously over the spirit which He has made...")
2 Peter 1:20 - interpreting Scripture is not a matter of one's own private interpretation. Therefore, it must be a matter of "public" interpretation of the Church. The Divine Word needs a Divine Interpreter. Private judgment leads to divisions, and this is why there are 30,000 different Protestant denominations.
2 Peter 3:15-16 - Peter says Paul's letters are inspired, but not all his letters are in the New Testament canon. See, for example, 1 Cor. 5:9-10; Col. 4:16. Also, Peter's use of the word "ignorant" means unschooled, which presupposes the requirement of oral apostolic instruction that comes from the Church.
2 Peter 3:16 - the Scriptures are difficult to understand and can be distorted by the ignorant to their destruction. God did not guarantee the Holy Spirit would lead each of us to infallibly interpret the Scriptures. But this is what Protestants must argue in order to support their doctrine of sola Scriptura. History and countless divisions in Protestantism disprove it.
1 John 4:1 - again, God instructs us to test all things, test all spirits. Notwithstanding what many Protestants argue, God's Word is not always obvious.
1 Sam. 3:1-9 - for example, the Lord speaks to Samuel, but Samuel doesn't recognize it is God. The Word of God is not self-attesting.
1 Kings 13:1-32 - in this story, we see that a man can't discern between God's word (the commandment "don't eat") and a prophet's erroneous word (that God had rescinded his commandment "don't eat"). The words of the Bible, in spite of what many Protestants must argue, are not always clear and understandable. This is why there are 30,000 different Protestant churches and one Holy Catholic Church.
Gen. to Rev. - Protestants must admit that knowing what books belong in the Bible is necessary for our salvation. However, because the Bible has no "inspired contents page," you must look outside the Bible to see how its books were selected. This destroys the sola Scriptura theory. The canon of Scripture is a Revelation from God which is necessary for our salvation, and which comes from outside the Bible. Instead, this Revelation was given by God to the Catholic Church, the pinnacle and foundation of the truth (1 Tim. 3:15).
"All Scripture is Inspired"- 2 Tim. 3:16-17
2 Tim. 3:14 - Protestants usually use 2 Tim. 3:16-17 to prove that the Bible is the sole authority of God's word. But examining these texts disproves their claim. Here, Paul appeals to apostolic tradition right before the Protestants' often quoted verse 2 Tim. 3:16-17. Thus, there is an appeal to tradition before there is an appeal to the Scriptures, and Protestants generally ignore this fact.
2 Tim. 3:15 - Paul then appeals to the sacred writings of Scripture referring to the Old Testament Scriptures with which Timothy was raised (not the New Testament which was not even compiled at the time of Paul's teaching). This verse also proves that one can come to faith in Jesus Christ without the New Testament.
2 Tim. 3:16 - this verse says that Scripture is "profitable" for every good work, but not exclusive. The word "profitable" is "ophelimos" in Greek. "Ophelimos" only means useful, which underscores that Scripture is not mandatory or exclusive. Protestants unbiblically argue that profitable means exclusive.
2 Tim. 3:16 - further, the verse "all Scripture" uses the words "pasa graphe" which actually means every (not all) Scripture. This means every passage of Scripture is useful. Thus, the erroneous Protestant reading of "pasa graphe" would mean every single passage of Scripture is exclusive. This would mean Christians could not only use "sola Matthew," or "sola Mark," but could rely on one single verse from a Gospel as the exclusive authority of God's word. This, of course, is not true and even Protestants would agree. Also, "pasa graphe" cannot mean "all of Scripture" because there was no New Testament canon to which Paul could have been referring, unless Protestants argue that the New Testament is not being included by Paul.
2 Tim. 3:16 - also, these inspired Old Testament Scriptures Paul is referring to included the deuterocanonical books which the Protestants removed from the Bible 1,500 years later.
2 Tim. 3:17 - Paul's reference to the "man of God" who may be complete refers to a clergyman, not a layman. It is an instruction to a bishop of the Church. So, although Protestants use it to prove their case, the passage is not even relevant to most of the faithful.
2 Tim. 3:17 - further, Paul's use of the word "complete" for every good work is "artios" which simply means the clergy is "suitable" or "fit." Also, artios does not describe the Scriptures, it describes the clergyman. So, Protestants cannot use this verse to argue the Scriptures are complete.
James 1:4 - steadfastness also makes a man "perfect (teleioi) and complete (holoklepoi), lacking nothing." This verse is important because "teleioi"and "holoklepoi" are much stronger words than "artios," but Protestants do not argue that steadfastness is all one needs to be a Christian.
Titus 3:8 - good deeds are also "profitable" to men. For Protestants especially, profitable cannot mean "exclusive" here.
2 Tim 2:21- purity is also profitable for "any good work" ("pan ergon agathon"). This wording is the same as 2 Tim. 3:17, which shows that the Scriptures are not exclusive, and that other things (good deeds and purity) are also profitable to men.
Col. 4:12 - prayer also makes men "fully assured." No where does Scripture say the Christian faith is based solely on a book.
2 Tim. 3:16-17 - Finally, if these verses really mean that Paul was teaching sola Scriptura to the early Church, then why in 1 Thess. 2:13 does Paul teach that he is giving Revelation from God orally? Either Paul is contradicting his own teaching on sola Scriptura, or Paul was not teaching sola Scriptura in 2 Tim. 3:16-17. This is a critical point which Protestants cannot reconcile with their sola Scriptura position.
Other Passages used to Support "Sola Scriptura"
John 5:39 - some non-Catholics use this verse to prove sola Scriptura. But when Jesus said "search the Scriptures," He was rebuking the Jews who did not believe that He was the Messiah. Jesus tells them to search the Scriptures to verify the Messianic prophecies and His oral teaching, and does not say "search the Scriptures alone." Moreover, since the New Testament was not yet written, the passage is not relevant to the Protestant claim of sola Scriptura.
John 10:35 - some Protestants also use this verse "Scripture cannot be broken" to somehow prove sola Scriptura. But this statement refers to the Old Testament Scriptures and has nothing to do with the exclusivity of Scripture and the New Testament.
John 20:31 - Protestants also use this verse to prove sola Scriptura. Indeed, Scripture assists in learning to believe in Jesus, but this passage does not say Scripture is exclusive, or even necessary, to be saved by Jesus.
Acts 17:11-12 - here we see the verse "they searched the Scriptures." This refers to the Bereans who used the Old Testament to confirm the oral teachings about the Messiah. The verses do not say the Bereans searched the Scriptures alone (which is what Protestants are attempting to prove when quoting this passage). Moreover, the Bereans accepted the oral teaching from Paul as God's word before searching the Scriptures, which disproves the Berean's use of sola Scriptura.
Acts 17:11-12 - Also, the Bereans, being more "noble" or "fair minded," meant that they were more reasonable and less violent than the Thessalonians in Acts. 17:5-9. Their greater fairmindedness was not because of their use of Scripture, which Paul directed his listeners to do as was his custom (Acts 17:3).
1 Cor. 4:6 - this is one of the most confusing passages in Scripture. Many scholars believe the phrase "don't go above the line" was inserted by a translator as an instruction to someone in the translation process. Others say Paul is quoting a proverb regarding kids learning to write by tracing letters. By saying don't go above line, Paul is probably instructing them not to be arrogant. But even if the phrase is taken literally, to what was Paul referring? The Talmud? The Mosaic law? The Old Testament Scriptures? This proves too much for the Protestant because there was no New Testament canon at the time Paul wrote this, and the text says nothing about the Bible being the sole rule and guide of faith.
Rev. 1:11,19 - Non-Catholics sometimes refer to Jesus' commands to John to write as support for the theory that the Bible is the only source of Christian faith. Yes, Jesus commands John to write because John was in exile in Patmos and could not preach the Word (which was Jesus' usual command). Further, such a commandment would be limited to the book that John wrote, the Book of Revelation, and would have nothing to do with the other Scriptures.
Rev. 22:18-19 - some Protestants argue against Catholic tradition by citing this verse, "don't add to the prophecies in this book." But this commandment only refers to the book of Revelation, not the entire Bible which came 300 years later.
Deut 4:2; 12:32 - moreover, God commands the same thing here but this did not preclude Christians from accepting the Old Testament books after Deuteronomy or the New Testament.
Now, about Oral Tradition:
The Word of God is Transferred Orally
Mark 13:31 - heaven and earth will pass away, but Jesus' Word will not pass away. But Jesus never says anything about His Word being entirely committed to a book. Also, it took 400 years to compile the Bible, and another 1,000 years to invent the printing press. How was the Word of God communicated? Orally, by the bishops of the Church, with the guidance and protection of the Holy Spirit.
Mark 16:15 - Jesus commands the apostles to preach the Gospel to every creature. But Jesus did not want this preaching to stop after the apostles died, and yet the Bible was not compiled until four centuries later. The word of God was transferred orally.
Mark 3:14; 16:15 - Jesus commands the apostles to preach (not write) the gospel to the world. Jesus gives no commandment to the apostles to write, and gives them no indication that the oral apostolic word he commanded them to communicate would later die in the fourth century. If Jesus wanted Christianity to be limited to a book (which would be finalized four centuries later), wouldn't He have said a word about it?
Luke 10:16 - He who hears you (not "who reads your writings"), hears me. The oral word passes from Jesus to the apostles to their successors by the gracious gifts of the Holy Spirit. This succession has been preserved in the Holy Catholic Church.
Luke 24:47 - Jesus explains that repentance and forgiveness of sins must be preached (not written) in Christ's name to all nations. For Protestants to argue that the word of God is now limited to a book (subject to thousands of different interpretations) is to not only ignore Scripture, but introduce a radical theory about how God spreads His word which would have been unbelievable to the people at the time of Jesus.
Acts 2:3-4 - the Holy Spirit came to the apostles in the form of "tongues" of fire so that they would "speak" (not just write) the Word.
Acts 15:27 - Judas and Silas, successors to the apostles, were sent to bring God's infallible Word by "word of mouth."
Rom. 10:8 - the Word is near you, on your lips and in your heart, which is the word of faith which is preached (not just written).
Rom. 10:17 - faith comes by what is "heard" (not just read) which is the Word that is "preached" (not read). This word comes from the oral tradition of the apostles. Those in countries where the Scriptures are not available can still come to faith in Jesus Christ.
1 Cor. 15:1,11 - faith comes from what is "preached" (not read). For non-Catholics to argue that oral tradition once existed but exists no longer, they must prove this from Scripture. But no where does Scripture say oral tradition died with the apostles. To the contrary, Scripture says the oral word abides forever.
Gal. 1:11-12 - the Gospel which is "preached" (not read) to me is not a man's Gospel, but the Revelation of Jesus Christ.
Eph. 1:13 - hearing (not reading) the Word of truth is the gospel of our salvation. This is the living word in the Church's living tradition.
Col. 1:5 - of this you have "heard" (not read) before in the word of truth, the Gospel which has come to you.
1 Thess. 2:13 - the Word of God is what you have "heard" (not read). The orally communicated word of God lasts forever, and this word is preserved within the Church by the Holy Spirit.
2 Tim. 1:13 - oral communications are protected by the Spirit. They abide forever. Oral authority does not die with the apostles.
2 Tim. 4:2,6-7 - Paul, at the end of his life, charges Timothy to preach (not write) the Word. Oral teaching does not die with Paul.
Titus 1:3 - God's word is manifested "through preaching" (not writing). This "preaching" is the tradition that comes from the apostles.
1 Peter 1:25 - the Word of the Lord abides forever and that Word is the good news that was "preached" (not read) to you. Because the Word is preached by the apostles and it lasts forever, it must be preserved by the apostles' successors, or this could not be possible. Also, because the oral word abides forever, oral apostolic tradition could not have died in the fourth century with all teachings being committed to Scripture.
2 Peter 1:12, 15 - Peter says that he will leave a "means to recall these things in mind." But since this was his last canonical epistle, this "means to recall" must therefore be the apostolic tradition and teaching authority of his office that he left behind.
2 John 1:12; 3 John 13 - John prefers to speak and not to write. Throughout history, the Word of God was always transferred orally and Jesus did not change this. To do so would have been a radical departure from the Judaic tradition.
Deut. 31:9-12 - Moses had the law read only every seven years. Was the word of God absent during the seven year interval? Of course not. The Word of God has always been given orally by God's appointed ones, and was never limited to Scripture.
Isa. 40:8 - the grass withers, the flower fades, but the Word of our God (not necessarily written) will stand forever.
Isa. 59:21 - Isaiah prophesies the promise of a living voice to hand on the Word of God to generations by mouth, not by a book. This is either a false prophecy, or it has been fulfilled by the Catholic Church.
Joel 1:3 - tell your children of the Word of the Lord, and they tell their children, and their children tell another generation.
Mal. 2:7 - the lips of a priest guard knowledge, and we should seek instruction from his mouth. Protestants want to argue all oral tradition was committed to Scripture? But no where does Scripture say this.
Learning through Oral Apostolic Tradition
Matt. 15:3 - Jesus condemns human traditions that void God's word. Some Protestants use this verse to condemn all tradition. But this verse has nothing to do with the tradition we must obey that was handed down to us from the apostles. (Here, the Pharisees, in their human tradition, gave goods to the temple to avoid taking care of their parents, and this voids God's law of honoring one's father and mother.)
Mark 7:9 - this is the same as Matt. 15:3 - there is a distinction between human tradition (that we should reject) and apostolic tradition (that we must accept).
Gal. 1:14; Col. 2:22 – Paul also writes about “the traditions of my fathers” and “human precepts and doctrines” which regarded the laws of Judaism. These traditions are no longer necessary.
Acts 2:42 - the members obeyed apostolic tradition (doctrine, prayers, and the breaking of bread). Their obedience was not to the Scriptures alone. Tradition (in Greek, "paradosis") means "to hand on" teaching.
Acts 20:7 - this verse gives us a glimpse of Christian worship on Sunday, but changing the Lord's day from Saturday to Sunday is understood primarily from oral apostolic tradition.
John 17:20 - Jesus prays for all who believe in Him through the oral word of the apostles. Jesus protects oral apostolic teaching.
1 Cor. 11:2 - Paul commends the faithful for maintaining the apostolic tradition that they have received. The oral word is preserved and protected by the Spirit.
Eph. 4:20 – Paul refers the Ephesians to the oral tradition they previously received when he writes, “You did not so learn Christ!”
Phil. 4:9 - Paul says that what you have learned and received and heard and seen in me, do. This refers to learning from his preaching and example, which is apostolic tradition.
Col. 1:5-6 – of this you have heard before in the word of the truth, the gospel, which has come to you. This delivery of the faith refers to the oral tradition the Colossians had previously received from the ordained leaders of the Church. This oral tradition is called the gospel of truth.
1 Thess.1:5 – our gospel came to you not only in word, but in the power of the Holy Spirit. Paul is referring to the oral tradition which the Thessalonians had previously received. There is never any instruction to abandon these previous teachings; to the contrary, they are to be followed as the word of God.
1 Thess. 4:2 – Paul again refers the Thessalonians to the instructions they already had received, which is the oral apostolic tradition.
2 Thess. 2:5 – Paul yet again refers the Thessalonians to the previous teachings they received from Paul when he taught them orally. These oral teachings are no less significant than the written teachings.
2 Thess. 2:15 - Paul clearly commands us in this verse to obey oral apostolic tradition. He says stand firm and hold to the traditions which you were taught, either by word of mouth or letter. This verse proves that for apostolic authority, oral and written communications are on par with each other. Protestants must find a verse that voids this commandment to obey oral tradition elsewhere in the Bible, or they are not abiding by the teachings of Scripture.
2 Thess. 2:15 - in fact, it was this apostolic tradition that allowed the Church to select the Bible canon (apostolicity was determined from tradition). Since all the apostles were deceased at the time the canon was decided, the Church had to rely on the apostolic tradition of their successors. Hence, the Bible is an apostolic tradition of the Catholic Church. This also proves that oral tradition did not cease with the death of the last apostle. Other examples of apostolic tradition include the teachings on the Blessed Trinity, the hypostatic union (Jesus had a divine and human nature in one person), the filioque (that the Spirit proceeds from the Father and the Son), the assumption of Mary, and knowing that the Gospel of Matthew was written by Matthew.
2 Thess. 3:6 - Paul again commands the faithful to live in accord with the tradition that they received from the apostles.
2 Thess. 3:7 - Paul tells them they already know how to imitate the elders. He is referring them to the tradition they have learned by his oral preaching and example.
1 Tim. 6:20 - guard what has been "entrusted" to you. The word "entrusted" is "paratheke" which means a "deposit." Oral tradition is part of what the Church has always called the Deposit of Faith.
2 Tim. 2:2 - Paul says what you have heard from me entrust to faithful men who will be able to teach others also. This is "tradition," or the handing on of apostolic teaching.
2 Tim. 3:14 - continue in what you have learned and believed knowing from whom you learned it (by oral tradition).
1 John 2:7 – John refers to the oral word his disciples have heard which is the old commandment that we love one another.
Examples of Jesus' and the Apostles' Reliance on Oral Tradition
Matt. 2:23 - the prophecy "He shall be a Nazarene" is oral tradition. It is not found in the Old Testament. This demonstrates that the apostles relied upon oral tradition and taught by oral tradition.
Matt 23:2 - Jesus relies on the oral tradition of acknowledging Moses' seat of authority (which passed from Moses to Joshua to the Sanhedrin). This is not recorded in the Old Testament.
John 19:26; 20:2; 21:20,24 - knowing that the "beloved disciple" is John is inferred from Scripture, but is also largely oral tradition.
Acts 20:35 - Paul relies on the oral tradition of the apostles for this statement ("it is better to give than to receive") of Jesus. It is not recorded in the Gospels.
1 Cor. 7:10 - Paul relies on the oral tradition of the apostles to give the charge of Jesus that a wife should not separate from her husband.
1 Cor. 10:4 - Paul relies on the oral tradition of the rock following Moses. It is not recorded in the Old Testament. See Exodus 17:1-17 and Num. 20:2-13.
Eph 5:14 - Paul relies on oral tradition to quote an early Christian hymn - "awake O sleeper rise from the dead and Christ shall give you light."
Heb. 11:37 - the author of Hebrews relies on the oral tradition of the martyrs being sawed in two. This is not recorded in the Old Testament.
Jude 9 - Jude relies on the oral tradition of the Archangel Michael's dispute with satan over Moses' body. This is not found in the Old Testament.
Jude 14-15 - Jude relies on the oral tradition of Enoch's prophecy which is not recorded in the Old Testament.
Hope this helps.