-In my opines, she doesn't have an obligation to stop the mother because she doesn't know the mother moreover she doesn't know that situation why her hit the child maybe the mother have a reason to do that and it's the mother right to hit the child.
-I think she have obligation to stop the mother because the mother punish too much with her 2 year-old child. Although she is the mother and have duty to instruct, punish her child but her should not do it this while her is very angry.