motdakasha

If you keep silent about it and he's found out by his wife. She will likely tell your wife and your wife will wonder why you didn't tell her. Which may or may not develop trust issues between you and your wife. I'm just saying that's a possibility.

If you are honest with someone, him or your wife, at least you made some effort and maybe you didn't taddletale, but you were at least trying to be honest. You don't have to announce to his wife or anything that he's having an affair, but if you tell someone, your ass is covered from his mistake. Think of is as a crime (and well, cheating is against the law). A friend tells you he murdered someone, he's caught in court. You get dragged into court because you've become an accomplice or conspirator to commit a crime or whatever. "Well, I didn't do anything." That's exactly why you've become a conspirator. So, anyways..
If you tell him, at least you tried to steer him in the right direction. If you tell your wife, at least you were honest with her. Even if you don't want to be a part of this, you are. You shouldn't just be concerned with the plight of your friend, but now you also have to concern yourself with how it's going to look when his wife does find out. It's up to you to do something good, if at all--or risk becoming a bad guy, too.

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=^-^= motdakasha =^-^=
Just Google It.
BA Psychology & Photography
(I'm not going psychoanalyze you nor will I let you cry on my shoulder. Have a nice day.)