I very much agree RedPepper, but it one of those sacrifice your integrity for your heart kind of things for her. I don't expect them to be an LTR.
He's been married for 10 years, and a cheater for 7. I think he got caught back in March, and has been on couch duty since. From what I understand, they are in it for their children still. I actually observed his interaction with his wife at an event last night, and there's no doubt in my mind that they are in a loveless, long since expired marriage. Evidently he has already advised my wife that he's in love with his (married) ex-girlfriend. She's ok with a sexual, dating, semi-romantic courtship; with the understanding that while there is romanance between the two, it's not love.
He's very un-skilled at the discretion of cheating, I'm confident his wife knows he's not faithful, and I'd be surprised if she is faithful as well.
This is no attempt at justification of his behaviour, hell, I'd be fine if he went away; just an attempt at clarification.