If you guys are really connected he'll likely know you are hiding something anyway. Imagine yourself in his shoes if he finds out later that you knew about the affair. You're friend has put you in a very tough spot. Walk away or tell him? If you tell him, you are impacting their relationship, if you don't you are lying by omission.
The option with the most integrity in my opinion is to get your freind to tell him. Let her be honest and and deal with the reality of her one sided relationship.
Playing the Game of Life with Monopoly rules.
Monogamy might just be in my genes
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