A bunch of boys hanging up to beat a girl? That is quite odd because most boys would be embarrassed.
the part where the girl afraid of him instead of thanking him, it hurt to watch but can't be helped since most of victims mind is working like that.
they have no courage to come out from their situation but has no regard on those who helped them.
not all of them like that but most case is.
another exampie is when bullied person fights back, they got punished instead. because People around them know but they didn't help so they deep down afraid to be held for responsible and accountable.
Actually, if the girl was afraid she would just agree passively but never actually call him. More logically is that she is afraid his meddling will mean those guys will take it out on her more harshly when he isn't watching so she is pushing him away.
Also, the big reason why a victim can get in trouble is because most bullies know where to draw the line not to get in trouble. A bullied person usually fights back at spur of the moment without thinking ignoring all lines.
Perception is important and many people don't consider that when they fling out emotionally