Well, my opinion is pretty much covered here. Really destructive idea. If I were you I would tell this person why I wouldn't involve myself with him as a heads up that he has major work to do and life changes to make. He is on a path to major destruction, pain, cruelty and a whole lot of craziness. Why not change it up and take the healthy root and get about fixing his relationship or leaving.
You might want to do a tag search for "cheating" there are some really good stories and discussions here that could help him and you.
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