Let me first of all commend you for your concern for your friend. You see what he cannot see right now: He's not only destroying his life, but he is in danger of losing his eternal soul. And you also know that Christ could change his life and give him the joy and peace he needs.
One of the most devastating effects of sin in our lives is that it deceives us and blinds us to its effects—and that is what is happening to your friend. The Bible warns, "There is a way that seems right to a man, but in the end it leads to death" (Proverbs 16:25). Tragically, the longer a person turns away from God, the more insensitive and hardened they often become. This is why the Bible warns, "Today, if you hear his voice, do not harden your hearts" (Hebrews 4:7).
Only God can break through the hard shell that sin has built up around his heart—and that's why the most important thing you can do is to pray for your friend. God alone can convict him of his sin, and God alone can open his heart and help him see why he needs Christ. The Bible says, "The prayer of a righteous man is powerful and effective" (James 5:16).
Then ask God to help you be a witness to him—not just by your words, but by your life. Does he see Christ living through you? Does he see Christ's love and peace in your life? Remember: Our lives often speak far louder than our lips.