It's a toughie - saying nothing will look to the innocent party like you've taken the side of the cheater.

I would go to the cheater first and insist that s/he confess/end the affair/end the relationship with the 'cheatee' - if not, you will. Admittedly this may damage your relationship with the cheater, but then what sort of friend allows you to end up in that situation? And do you really want a liar and a cheat as one of your friends in the first place?

I know it sounds harsh, but I come down on the moral side with things like this. The innocent party deserves your loyalty more than the cheater.

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