WTF!!!

so me my husband and my son Joseph went to my grandmother's house because she wanted to see us she is 81 and we do some household chores while we are there to help her out. me and my husband went out to run a few errands and left Joseph with my grandmother when we got back about 1 hour later we came through the door and I hear a slap. I RUSHED to the living room and there was Joseph on the floor crying because my grandmother spanked him on the butt 3 times and then when he fell on the floor slapped him on his forehead. I am BEYOND pissed at her and will probably not be going back to her house to stay with her anymore. I ran in the living room and screamed STOP!!! STOP!!! WHAT ARE YOU DOING!!! and she said he was crying and it got on my nerves I said that is NOT how you treat a child or any living thing. and just walked out of the room. I am LIVID with her at the moment. I don't know what to do.