C.O.
My oldest son is a black belt in TKD - it would have been his DUTY to come to her aid.
My youngest son is a high Blue Belt in TKD - again - his duty to come to her aid.
Either way? I would have expected my children to help the girl or a boy if they saw something like that happening.
What I DO know my son would do is stick up for the girl. He has in the past.