Like said above - there actually is research out there showing that those forms of venting anger actually help deepen the anger.

But to say that girls shouldn't be like that?! WTF! That's ridiculous.

If you're looking for an alternative, have you thought about martial arts? There is often a spiritual sort of element to it that could help balance the anger with learning how to work with the emotions.

Does she like art at all? It's also a good way to express anger.