I Said: From a very practical standpoint, if someone wants to marry him, then I don’t see any legal or scriptural ban on it. Now, to be honest, if he were trying to marry one of my dear female friends, then I would be skeptical at first. But the Lord can redeem people and change them.
The interesting thing about your question is the caveat that your friend was a Christian when he cheated. There is a very prevalent undercurrent that Jesus will forgive and redeem your sin, as long as your sins happened or at least started before you were a Christian. So this says, without outrightly saying it, that Christians shouldn’t screw up. And if they do, Jesus holds it against them because they should have known better. God is outside of time. When Jesus died on the cross for your sin, they were all future sin to him. Romans 7 is all about Paul wrestling with his sin, and admitting to sometimes losing, well after he had seen the Lord in person.
Just as there is no sin that is beyond the forgiveness of God, there is no circumstances around a sin that renders it beyond God’s grace.