Science is a particular method for finding out what's true, and it only works for some cases. (You have to be able to formulate testable assertions and then make observations that are complete enough to establish whether the assertions are true.) The cases for which science works cover a wide range of topics, but not all possible questions that might be worth considering.
Religion is a lot of different things, but mainly it's an expression of values (either personal or cultural). It combines many methods, from simple assertion of assumptions to telling stories and then discussing what the stories might mean. It's generally a pretty crappy way of approaching any question that can be answered by science, and usually people who claim science is wrong and religion is right are mistaking the details of the stories for their meanings.
When your friend says religion is the "why," I think he means that religion gives us a way to express values and meanings that are utterly out of the reach of science. They're too personal for science to sort out by any experiment. But science is pretty good at answering questions of "how" something could happen.
If a person makes that mistake I already described, where he confuses the story details with the meanings, he's likely to think he has religious answers to scientific questions. Evolution and the Big Bang are the ones most frequently debated: science has collected evidence that both happened, but the religious stories don't mention either. A person who has mistaken what religion tells us, and who really, really believes his mistakes, can find his faith challenged by scientific fact. A person who avoids that mistake doesn't have that problem.
Some atheists agree to the mistake: they treat the stories as though their details are supposed to be a science text, and therefore (of course) a text that's wrong about a lot of things. For some reason, both religious fundamentalists and atheists tend to read religious texts that way--as though the Bible were written on the level of "See Spot Run." Anybody who has progressed to "The Cat in the Hat," and beyond, really ought to know that most stories are NOT intended to be read that way. But they insist the Bible is, and so they agree on something more important than their disagreement. They argue over whether it's literally true or literally false, and fail to consider that it's not meant literally at all. (As one religious studies professor used to tell us, in proportion to how literally take it, you miss the point.)
Actually, I find that the Garden of Eden story, when read as a myth, tells us very much the same story about the development of the human mind as Richard Leakey's "The Origin of Humankind" (a science text). Religion and science agree quite a bit on what works, and what doesn't work well, in human intelligence--which is, really, the whole topic of the Bible, though it's discussed in terms like "sin" and "justification" that make unsophisticated readers think it's somehow magical.