Mom smacked 2 year old in face


So today I was at my parents camper and my son was with me. He is 2. My mom took something from him & out of frustration or anger he bit her in the hand. This was the second time he had done so. I was literally standing right in front of them. Well my mom smacked him right in the face, so hard it made a loud smack noise and he had a hand print on his face. My mom instantly felt bad and mentioned that she hit him too hard but I felt sick to my stomach about it. She had apologized and obviously felt bad. I even looked at her and said you hit him so hard there’s a mark! It’s done and over with but now anytime I think about someone else hitting my child so hard it leaves a mark, I feel sick and it makes me feel horrible. What should I do? Ask my mom not to discipline him this way? I personally don’t spank him because he doesn’t respond to it. I have and he literally laughs at me. I do time outs with him and redirection. Any advice? Idk what to say to my mom. We’re actually a close family also. The other thing is I know she would NEVER hit my nephew (brothers son) in the face like that let alone spank him. It grinds my gears knowing she’ll do it to my son but not my nephew (they are 3.5 m apart... nephew is spoiled and babied bc he has a epilepsy disorder).