Fox, you need to remember that these MLCers are in an identity crisis! Rescuing someone is fun while it is a fantasy. And for a while thereafter while they feel like the saviour, but that is especially while the OW is still an affair, she is unknown except for what she says, she is wonderful, she is perfect. In reality she is far from perfect, we know it, they can't see it. Yet. But as living with them becomes the norm, they become less and less saintly, and gradually less and less perfect. What the MLCer doesn't want to let go of is the fantasy, the high, but eventually they will have to concede that the reality has not panned out the way they planned. OW is human, she burps and farts. And emotionally (and yours is a serious case in point) they are very damaged people. My H thinks he rescued his OW from her awful boyfriend. Boyf can't have been that awful because OW didn't actually leave him and move out until H had DEFINITELY left me and was looking for his own apartment, not that she uses men or anything. She had a job and a mother and, apparently, a friend she could have moved in with if she hated little old boyf so much (i kid you not - H told me she lived in the same town she grew up in and she had "a friend" - singular, sounds like she was loved by everyone - I am sure that the fact H felt she was "misunderstood" and he was the only one who could understand her added to her appeal) misunderstood/disliked - potato/potahto, semantics.
Anyway, my point - all is not as it seems, and all is definitely as a your H says. But even if he can't find the strength to escape, he will never be content. So, what choice do you have but to let go? Let go for your own sake, and don't worry about the results, you will survive and, indeed, thrive if you concentrate on your own well-being!