My partners mom hit our dog while petsitting, how to approach her?

Basically the title, me and my partner went out of town last week for a few days and my partners mom stayed at our house to watch them for us. Well today we found out from our roomate that she hit one of our dogs to "discipline" her at one point while watching them. As far as he knows it was only one time, but that is far from ok. Any advice on how to best approach her about this? We feel very betrayed since we've told her before that is not something we are ok with and how harmful it can be to dogs so she absolutely should have known better. She just tends to make herself the victim in any situation where she receives criticism and I'm not sure how to get her to understand that it was wrong and why were upset with her.