First, why do you fear so much your friend studying out of the bible the word of God, if you claim to be Christian as well? It sounds kind of moronic (not calling you, but saying your statement does) if you ask most, with common sense.
The best way is to lead by example. If your religion and way of life is the epitome of how Christians are commanded to live, then your friend will no doubt see the difference seeing he/she is studying from Gods word no matter who's doing the studying.
Trying to make someone see things your way is a dangerous approach, especially when it comes to ones religion. Hence why you should show them from Gods word, that backs up your choice of religion and way of life. If it's TRUTH then it can't be denied! And if they do deny it after you have proven what you believe to be the TRUTH of God, then ultimately they have to answer to God for it if in fact it is truth.
And like others have stated, maybe you should find out what JWs really believe and not base your opinion off of hear say. Because I'm sure if someone doesn't know much about your religion they could possibly form wrong opinions as well. Agreed?
ADD: I see you responded and can personaly say, that your question did not offend me in any way. Many ppl want to know how to counter "witness" the JWs. It's a common question on R,S.
If I'm to understand your response correctly, you basicly want to prove that what your friend is being taught is not scriptual, and want to know how to prove them wrong or "counter witness" correct? Well if it's a case of your friend trying to witness to you, just simply tell them you're not interested in it, and wish that they no longer discuss the JW religion. If that's not the case, and you want to show them the light so to speak well....
Without offending the only way to do this is by regular personal bible study on your part. That way when you do "evangalize" your "convictions" you'll be better equiped to do so. You very well can't tell or prove what someone is learning from a particular religion is wrong without knowing the scriptures yourself. 2 Tim 3:16,17-
ALL SCRIPTURE is inspired of God and BENEFICIAL FOR TEACHING, FOR REPROVING, FOR SETTING THINGS STRAIGHT, for disciplining in righteousness, 17 that the man of God may be FULLY COMPETENT, COMPLETELY EQUIPPED for every good work.
[* I emphasized the words that related to your question]
Sorry but that's common sense, and something a woman (solely going off of your avatar) your age SHOULD know.
Look at it like this. Lets say you work for a company and one day your sitting in on a staff meeting. Some guy/girl says to the boss "you know I have a perfect idea of how to run this company", and proceeds to explain how. Well you strongly disagree with everything that staff member suggested, and your afraid the boss might actually be leaning toward their idea. So what do you do?
Wouldn't it be the wise thing to investigate whether what the other staff member said was true or not? Crunch the numbers so to speak. How else are you gonna convince the boss that to look at it your way, if you know absolutely nothing about the co workers ideas, or even worse the company you're representing. How are you gonna persuade the boss to see things your way? No doubt you know you're gonna look VERY foolish if you're illprepared.
So the same holds true if your trying to "counter" ANYTHING. YOU...have to be prepared! If what you are trying to convey isn't important enough for you to have a deep understanding of it, how are you going to convince someone else to have the same conviction?
I bet you never thought when you asked this question, that a JW would be telling you how to "counter-evangelize" the JWs,LOL=)). Which isn't what I'm doing, but just trying to get you to see things in a different light. I wish you well wyomugs because trying to convince us JWs otherwise is a hard fought uphill, dang near impossible battle.=)