Even if he doesn’t realize it or want to admit it, your friend is on the road to almost certain disaster. Situations like this never end well, bringing heartache and guilt and mistrust in their wake. No wonder the Bible warns that the way of the adulterer “is bitter as gall, sharp as a double-edged sword” (Proverbs 5:4).
Can you make your friend face the folly of his ways, and get him to turn from them? Perhaps you can (with God’s help)—although he alone must make that decision. But even if he resists, you have a responsibility to try to help him, for unless you do, he’ll probably continue on his present course as long as possible. The Bible says, “Brothers and sisters, if someone is caught in a sin, you who live by the Spirit should restore that person gently” (Galatians 6:1).
Pray for your friend, that God will convict him of the wrongness of what he’s doing, and cause him to repent of it. Pray too that you will have opportunities to share your concern with him—not just telling him he’s wrong, but that you care for him and are concerned for his future. The Bible says, “A gentle tongue can break a bone” (Proverbs 25:15).
In addition, assure your friend of God’s promise of forgiveness and new life, if he will only repent and turn back to Christ. It won’t be easy, but with God’s help his marriage can be restored and strengthened.