I wanted to respond to your post because I have had problems with my mother-in-law too, although not involving physical violence.
It's important to keep in mind that your husband WILL always feel some loyalty to his mother and it may cause some disagreement later. NEVER believe that it is over and you are both unanimous in your decision to cut off the relationship (or whatever actions you have decided on). As time goes on, the severity of the incident will fade in his mind and he will begin to feel guilty about it. It's human nature. It has happened to me on numerous occasions with my sister-in-law whom I have refused to believe is as damaged as she would have everyone believe, regardless of the times she has hurt me personally.
My husband prefers to keep his family at arm's length while they continue to badger him with calls when they have decided he has done something they disapprove of. In the beginning, I would put in my two cents worth on his family's behavior but I have found that doing this only causes disagreements between us. He's the only one who can say these things about his family because they are HIS family and not mine. Chiming in with the insults will only trigger a defensive argument.
I think it is important that the decision be made MOSTLY by him because later, if it was YOU who dominated the decision to cut off the relationship, some resentment will emerge.
While I have never met your MIL, I have discovered, over time, that there were situations that I exacerbated with my stubbornness and combativeness (I was a new mom and had just come from two terms in the Marine Corps so I was pretty terrible). It took several years and much soul searching for me to realize that there were some things that weren't that important for me to be right about. I knew I was right but it wasn't necessary for me to prove to her that I was right.
Anyway, your situation may be totally different but the point is that your husband should not feel pushed into the decision to alienate his mother or else it might come back to bite you in the bottom later.
Having said that mouthful, no violence is acceptable. An adult with grandchildren should be old enough to know that. Shame on her.