I also have the MIL from hell. Opposed our marriage, still doesn't pretend to like me. Has done her best to undermine me, etc. The positive thing is that my WH NEVER tolerated that crap. However I do want you to consider some thoughts:
One of the things WH has spent a lot of time thinking about in IC is the fact that he never openly rebelled from his mother's controlling ways when he was a kid. He just went along with it, and internalized the message that her constant picking at his behavior meant that he wasn't good enough. His way of dealing with this was compartmentalization. Your WH sounds like his mechanism is conflict avoidance. Both of these things are part of the reason behind an A. (We've talked a lot about how much the OW actually looks like his mom, gross as that is.)
Gotta go wake the kids up, but I do want you to push this a bit further. Your WH has to change his behaviors as a result of the A, and this is a really good place for him to start. He will likely feel MUCH better. As horrible as his mom's behavior is to you, imagine being brought up that way! This likely brings up so much inside him, and if he can resolve it, that will be a big step.