Did it happen only once and he fully admits it and does not do it anymore? Then forget those that have given him over. I am sure God has forgiven him. Just encourage him, in the name of the Lord.
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It is absolutely essential that the repented sinner is
really repented and willing to not cheat on their spouses anymore. I know many church goers who go to bed with people other than their spouses and then just say: "Oh Lord, I'm sorry". They claim that the Lord Jesus has already paid for
ALL our past and future sins, but here is a difference between sin and sin. We are all sinners, but we are not all adulterers, fornicators, thieves, idolaters, slanderers, liars, etc.