XY,
I hope I don't incur your wrath by saying this, but I think your hubby is not very tactful. His mother took the effort and time to care for his boy and felt treated like a maid. Obviously she must have felt unappreciated. He shouldn't have said she's free to go if she felt this way. It's very hurting.
My bro-in-law knows he's under-paying my mother for taking care of his three very active kids, so he will buy things for my mother on adhoc basis to make my mother feel appreciated. I think that's what we call '会做人'.
By硬碰硬with his own mother, he is in a fix. If he apologises to his mother, she might think that her threat works. Next time she will employ the same tactic; if he refuses to apologise, she won't come back. And if he cancels her social visit pass, it might backfire and worsen the relationship. He should pay his mother a visit and explain his outburst I think.