Only God knows your friend's heart, and whether or not he has truly given his life to Jesus Christ or is only going through the motions. Few people are in greater spiritual danger than the person who thinks he or she is a Christian, but in reality is not.
When we sincerely come to Christ, however, our lives will be different. We won't live the same way we once did -- and the reason is because Christ has come to live within us by His Holy Spirit. Our sins have been forgiven, and God has set our feet on a new path. This is why the Bible says, "If anyone is in Christ, he is a new creation; the old has gone, the new has come!" (2 Corinthians 5:17). If our lives aren't any different, we must ask ourselves if our commitment to Christ is real.
Does that mean we'll be perfect, or that we'll never sin or drift away from God? No, of course not. Peter had sincerely confessed Christ as his Lord and Savior -- but after Jesus' arrest he denied he even knew Him (see Matthew 26:69-75). But Peter repented of his sin, and later he was greatly used of God to proclaim the Gospel.
Pray for your friend, and ask God for an opportunity to confront him about his failure to live for Christ. But examine your own life as well, and ask God to help you to be a consistent follower of Jesus every day.