Would you consider this inappropriate touching?

I have a friend and her step dad thinks it is ok to tap her on the butt - he calls it the baseball slap - anytime she does something good. Somehow it seems she's always doing something good cos I see it everytime I am over her house. She brushes it off and got mad at me when I told her I thought it was weird. She's 16 and I know she thinks he is good looking cos she told me this. I don't think they are doing anything cos that would be super weird but shouldn't there be boundaries? Should I tell her mom? It creeps me out to see. She also has a boyfriend and I told him. He never saw it but that's cos her dad avoids doing it around him so he doesn't believe me. What should I do????