I shared this exp with a couple of my female teacher friends today, and one teacher said she is "always afraid" to say anything, and related that she has friend who is a counselor who will intervene kindly, saying something like, "I see you're having a hard time here, can I help you?"
Personally, though, I don't think the kind approach would have the impact of a "verbal slap" when a person is beating a child in public!
I got hit, switched, shaken and smacked daily by my mother from very early. I remember wishing my father, who never hit me, would intervene, but he never did. The only thing I remember is one time his saying, "You're going to give her an inferiority complex!" I've never forgotten that, and perhaps it made an impact, like hearing that it's not okay.
As a parent of one daughter, I never ever hit her, but used talk and reasoning instead. One time, after a counselor had heard of my childhood, he stated, "You hit your daughter, don't you!" to which I replied "No, I've never hit her!" He appologized during our next session.
Patty