Brains aren't fully formed at that age.
Add alcohol into the mix, and you get issues like this.
She should have never, under any circumstances put her hands on him.
However, you don't kill mosquitoes with bazookas.
A firm but calm hold on her while explaining to her that you don't put your hands on people would have sufficed.
I bartended at a number of places. Girl fights were the worst. When you are breaking one up, they are scratching at everything.
With that being said, they have a buzzkill moment when they realize they are severely overpowered and that the police will be here to arrest them on assault charges very soon if they don't act correctly.