I would tell him he needs to talk to his mom...but that is pretty messed up of her.
I had a crazy mother in law situation.. Dealt with it for a decade.. My advice to you concerning this and all other things that will come up is stay out of it 'as hard as it may be at times' doesn't matter if you mean well or are trying to protect hubby or diffuse a situation and feel you need to say something it will always end up being a bigger issue than it was to begin with, so the best thing for your marriage really is just simply stay out of it and always be supportive of your husband..
But then he will know that I knew and will be mad I didnt tell him.... Its like a rock and a hard place....
I think that if you break it to him, the potential for issues grows. I think its his mom's responsibility to tell her son. Because then she will have to explain why she won't be there. Just be there to support him.