One of the Bible's greatest truths is that no matter how bad we are, God accepts us just as we are. Not only that, but God also is able to change us, because He is more powerful than sin and the devil.
One of the best examples of this is the Apostle Paul in the Bible. Do you remember his story? He has rightly been called the greatest Christian who ever lived - but he wasn't always that way. In fact, at one time he violently hated Christians, and he dedicated his life to stamping them out - even if it meant putting them to death. But then he met Christ, and from that moment on his life was radically changed. Later he wrote, "If anyone is in Christ, he is a new creation; the old has gone, the new has come! All this is from God" (2 Corinthians 5:17-18).
Right now, your friend can't imagine why God would have anything to do with him. But the most important thing you can tell him is that God loves Him - and the proof is that Jesus Christ was willing to sacrifice His life for him. No matter what his past has been, God wants him to be part of His family forever.
Pray for this person, and ask God to help you show Christ's love to him. Challenge him also to face his need of God, and not to use either his past or his present as an excuse to keep God away. Jesus' promise is true: "Whoever comes to me I will never drive away" (John 6:37).