Speaking from someone who had a mom who always spoke her mind and made people feel uncomfortable a lot, (especially with kids it is worse as they say the stupidest things sometimes) I wanted to just say that first I always made it my problem with my mom and not my spouse. It should be your husband that should be talking to his mom to understand and it personally sounds like he needs some juevos (Spanish term). I never could understand wy so many guys are afraid to stick up for their wives to their moms. It is like hey you married this women because you loved her and of course no women is ever going to be good enough for a mom's son so the husband always needs to be the buffer.
Don't get the wrong idea, I love my mom and miss her everyday and I think you can stick up for you wive and still love your mom. Just don't get it.