90 year old grandmother abusing my cat?

Its never ok to hit a cat. The only thing it learns is to hate you. It cannot understand what to do like a child can. Since your grandma is only staying one ight and is probably senile and probably wont listen to you its best to just take the cat and put it in a room away from her with the litterbox and food and water. Than when your grandma leaves let the cat out. Im not trying to be mean but your grandma sounds freaking insane.