My ex's mom slapped Bug for swearing at her too. He was about 8. I told her that if it ever happened again, I would call the police and press charges for assault.
I would not throw your mom out because of this one incident. Having someone screaming in your face and swearing *is* very triggering, and I think she deserves forgiveness for it. I would possibly bring her along to your dd's next therapy session so it can be addressed and I would have your mom read some books that you choose to help her learn how to better deal with it.
I would make it clear that if it happens again, she will need to find a new place to live. She can file bankruptcy, use government assistance, find a roommate, etc. If she wants to live with you, she needs to realize that her behavior matters.