Atheism isn't as bad as everyone thinks. It is just the lack of a religion. And it does not mean your friend is a bad person or had a tragic experience like some people believe, he just doesn't believe in any religion. So overall, just be his friend, and if he is your best friend, it shouldn't be a problem.
I see nothing wrong with it, I'm atheist and I have plenty of friends in all religions and they all accept me for it. Why wouldn't you do the same? He's still the same person just with different views on religion.
You have to respect what and who he or she is if he or she is a good friend that's what count. We all live with difrent culture and religion and race accepting some one for who he or she is very greatfull. Be good stay Bless :-)
I'm 15 and have 3 friends who are atheists and I believe in God and Jesus Christ but I don't care if their atheists or not they are free to believe in what they believe in their just normal people too.
Christians must be willing and bold in our actual sharing of the gospel. Proclaim the message of salvation through Jesus Christ to your friends and family (Romans 10:9-10). Always be prepared to speak of your faith (1 Peter 3:15), doing so with gentleness and respect. Ultimately, we must leave the salvation of our loved ones up to God. It is God's power and grace that saves people, not our efforts. The best and most we can do is pray for them, witness to them, and live the Christian life in front of them.