My friend told me today he no longer believes in God or Christianity. I'd like to think I had some doing in this, of at least laying the seeds of doubt in his mind. A decision of that sorts is almost never because of an argument with a friend, though.
Is your friend okay? Has his shift in worldview caused him any depression? If you're a good person you'll keep your eye on him. These things can really destroy a person, if religion was an important part of their life.
Also, hooray for another person rejecting falsehood.
This has been a very long process, unlike mine where one day I was like " **** God, dude " and kind of became more of what Nietzsche thought the ideal man was: a dick.