I’m going to disagree with the majority here, who seem to be saying “flee from them!” And here’s why:
I was raised marginally Catholic. At some point in my early adult life I decided that I was an atheist, and I lived my life accordingly. While I felt that anyone had the right to believe what they wanted, I had a dislike for those in-your-face Bible-thumpers. it worked out okay, though, because they didn’t really want anything to do with me.
Eventually I had a close friend who was a deeply convicted Christian. His mere presence was a silent evangelization. They way he lived his life. The way that he would share his struggles with temptation. He never preached to me, but he never gave in on his morals, either. Eventually, when I had questions about the faith, I knew that I could ask him.
That friend, just by being my friend, had a lot to do with my return to Christ.
So I say, stay in your friend’s life. Lay the foundation for his repentence.
But… never give in on your principles! If he brings up his lifestyle, you have to say that it’s wrong! If he asks you to support it with some word or action, you can not.
Also, you do need to watch out for yourself. If he becomes a corrupting influence on your life, then, yes, you should end it. But if your faith is strong enough, be an example. Someday maybe he’ll be posting about how your example saved him!