This is a rather different question, but yes, I think that's morally questionable.
Yes you guys are. If you aren't capable of informing a " friend " that his missus has been sneaking around behind his back then what sort of friend are you to have? Clearly your "friends" missus doesn't give a flying **** about your "friend"
It would seem that way, wouldn't it.
Our mate cheated on her a lot prior to this. Once, even, when she was laid up in hospital for the night after going overboard on the booze. That seemed to be how they rolled (even if neither actually knew exactly what was going on).
Leave it up to the people in the relationship, is my opinion. They're the best judges.