I'm with the dad. I've done something similar minus the language. I got on the bus one morning and asked my daughter to show me who was bothering she and my son on the bus. I then told the bus driver that I would not tolerate this behavior and that I would be speaking with the principle. I also told her that I would not allow them on the bus again. The kids were called into the office and spoken to. Not sure what happened after that but mine haven't been back on the bus.
I realize that the driver has no way to supervise kids while she is driving which is why my kids aren't on the bus. There's no fucking way she can do both!
I will also say that bullying in schools hasn't gotten better. Most of the teachers don't give a shit so I have to rely on my kids to tell me what's going on and so far thankfully they do. This year we're lucky because my older two have excellent teachers but who knows what will happen next year.
Anyway I wish the best for this man and his daughter. I hope there is some way they can get her off the bus and drive her to school. It's so hard to see your kids suffer. It's instinct as a good parent to defend your young!