Unfortunately, I think yer friend is at the end of a bad road. I bet this girl loves excitement and controversy. Messing around, or at least the thought, gives her excitement. Plus her husband probably doesn't pay enough attention to her (she likes the dangerous, alloof, mysterious guy) and getting her rocks off on yer friend, and then doubly so when her husband finds out. He tears through the house pissed off, shows how much he's jealous, and this is all a rush for her.... mean while yer friend gets the stab from the husband, and nothing but heartache.
In short, she's using him and he is discardable. unless he seriously wants to destroy his life, I'd drop her like a cheeseburger with a pubic hair sticking outta it....