eeeww. what a prickly situation. although i agree with what uni said i have also been on the other end of that stick, albeit not in a work situation...but still. my ex-husband had an affair and noone told me. people around me knew about it and had the balls to look me in the eye and pretend not to know anything. it's a bad feeling knowing that everyone is keeping a secret from you. considering the fact that this is a work situation i would have to agree that your friend needs to keep out of it. BUT, the boss and friend brought her into it the moment they decided to the wildthing in the same room as her (whether they thought she was sleeping/drunk or whatever, it was inappropriate of them in soooo many ways to do that to her)
Hell no your friends 1st obligation is to her family. And in this economy she can't afford to be searching for employment. I'd play dumb and stay out of it. If I were the wife I'd be upset but I'd understand. Either way the wife will be taken care of but what about your friend? No I'd say I don't know anything and if you suspect anything you obviously know where we work and what time Quitting time is. This is also what a private Dick is for.
Sad to say but affairs of the heart are messy and no sense getting caught up in what is NOT your business. She's not doing him the other girl is.