God gives us all a choice and if a friend of mine decided that he/she doesnt believe in god anymore that is their choice. I would do my best to try and figure out why the change, ask questions, talk to my friend. Something could of gone wrong in their life, a prayer wasnt answered or my friend no longer felt God. My friend could be depressed, spiritually empty....something caused my friend to go from believing in God and to becoming a non believer. I would still be their friend, but I would be sad. I would try to bring them back to God, but I would not be in their face, pushy because that will cause them to back away further.