Would you tolerate this?

No one should have put their hands on her regardless of the problem. Job or no Job physical contact should have never been made, if the boss had such a big issue with her they should have fired her not smacked her. The authority positions will not gain respect by hitting her they should actually lose respect. What they did could be considered ault. No boss should ever physically punish or hit one of their workers, she should have been talked to like a mature adult. I would confront the boss, talk about the problems she is having with not "getting it" and also tell them she does not want them hitting her. What they did was out of line. I would possibly even quit.