If you had a good friend whose partner was having an affair you know, they dont do you tell them? Put this quandary to a dozen other friends and the responses may be contradictory and unhelpful: Stay out of anyones private business. I would tell. I would feel totally betrayed if the boot was on the other foot and they didnt tell me. Dont tell. If you do, you will be blamed. Who wants to be responsible for ending a relationship. Tell for sure, Id rather be told by the friend, than find out she knew and said nothing.