I've been in a similar situation to this with a boyfriend- he was doing all kinds of stuff behind my back, was so blatant about it I was suspicious and almost found out several times, all our mutual friends knew about it, but none of them told me they'd known until long after I'd dumped the guy anyway and he told me about it himself. I would have dumped the guy a lot sooner if I'd gotten a little push from somewhere, someone else confirming what I'd already been thinking. I wish one of my lousy friends at the time had the 'nads to risk "hurting my feelings" and tell me something I'd already suspected so I knew it wasn't just my imagination. A lot more than my stupid feelings could have been hurt if the guy had become a disease vector and I was unknowingly exposed.
I really think you should spill the beans so she can dump the mo fo already. She's just going to be miserable if she stays with this guy, the relationship obviously isn't supposed to be an open one, he's trampling her feelings and sees no problems, etc. After a while she'll build up enough resentment and make him miserable in turn for being oblivious. It's headed for divorce, whether it be tomorrow or ten years and three kids later from then. She's probably got her doubts about him already but is trying to write them off as paranoia or jitters on her part, give him benefit of the doubt since she loves him, and is afraid to make a jealous witch of herself if she's wrong. She won't come to her senses until something big falls in her lap. This is why she made the mistake of marrying him, despite all the clues about what he's like.
I could be completely off base with this, but there's my two cents based on what I've read.
