You’re right: Your friend is heading down the wrong road, and some day it will end in disaster. At the moment, he can’t see it (or he refuses to see it) — but it’s still true. The Bible warns, “There is a way that seems right to a man, but in the end it leads to death” (Proverbs 14:12).
What will likely happen if no one confronts him? Will he wake up, turn away from this illicit relationship and restore his marriage? Probably not. Only when he’s faced with a collapsing marriage and a desperately-hurt family will he realize what he’s done — and then it will be too late.
Instead, if you are the best person to reach him, I hope you’ll have the courage to confront him and urge him to turn from his sinful ways. He may reject you — but sometimes we have to take that risk in order to help someone. The Bible says, “Brothers, if someone is caught in a sin, you who are spiritual should restore him gently. But watch yourself, or you also may be tempted” (Galatians 6:1).
The most important thing you can do, however, is to pray for him. Only God can convict him of his sin, and only God can cause him to repent and turn to Jesus Christ. The Bible says, “Pray for each other so that you may be healed” (James 5:16).