I would let the cheater know that I knew, but I wouldn't say a word to the partner/spouse. Oh, I'd feel sorry for her/him, but in the end it's better for our relationship, as friends that they're not told. Even if you had solid proof, I still think it would hurt the friendship. I think there would be a degree of embarrassment and humiliation involved, knowing that I knew that there was cheating going on. Am I chear? I feel like I'm rambling.