Is something going on that’s putting this on your wife’s front burner? (Child-care frustrations, a health scare for Mom, a change in her retirement timeline?)
Perhaps in the ensuing months as she thinks things through, her views will change. When the time does come to discuss it – and it should be before that moving van is booked, of course – you should express what you told me. You want her mom close by, but there are many gradations of what “close by” can mean, and you want to make the best choice for all of you. Added brownie points: Be willing to explore those gradations – an English basement of a larger house, for instance.
I’m sure you’ve found that “Oh, he can’t be that bad” or “You should hear what MY husband does!” only add fuel to her fire. So, just disengage. Literally say nothing, or change the topic, denying her the positive reinforcement she so craves.
I should add, though, that looks can be deceiving. Is there any chance at all that she is in an abusive relationship? Or is this more of the garden-variety “My husband blows his nose in dirty laundry“? Either way, you’d do her some good to suggest counseling. Suggested wording: “A friend of mine was going through similar stuff, and she started seeing a therapist and found it to be a big help.” (Hey, imaginary friends are great fun.)