When he does that she comes across as very understanding (to DH) - it's ok, you're busy, don't worry about it, etc, etc. That's usually when I get thrown under the bus without his realizing it. Especially when he says we had dinner with my mom instead - who in reality said she would plan her dinner around whatever MIL wanted to do. She's aware of the tension and was trying to make things easier. MIL thinks I'm making DH go with my FOO, because he won't tell her he just doesn't want to drive.
I wish she would let into him about it, because it's not a nice thing to do. I think next time this comes up I'm going to have to say something. She probably never will, and I'm not sure he'll be convinced that it hurts her feelings. She's big on putting up a happy front to DH. When I told him he's hurt her feelings before (she looked absolutely crushed when he told her that DS is quite bonded to my mom) , he just couldn't see it. I think it's way easier for her to take things out on me instead of him.