The majority of players I have played with have a very hard time playing anything but themselves. That doesn't mean that they are bad roleplayers, just that they have somewhat limited range. So, if your friend has a hard time becoming truly violent in her characterization, it on the whole says good things about her. It's also not at all unusual. I'd say 80% of role players have a hard time playing someone with radically different emotional structure and beliefs than themselves (and generally have less fun when they try to).
First, let me say that before their were role playing games, if you looked up the term 'role playing' in a suitably broad dictionary or encyclopedia you would have discovered that the term was almost exclusively used to refer to psychology treatments intended to help a patient modify their real behaviors and attitudes. I don't want to sound like Jack Chick here, but it can't really be helped, so here goes: do be careful with this sort of thing. If the player has a hard time modifying their role play to act out anything other than themselves, be careful about how you push them. Role playing can if you are not careful become an unguided inadvertant behavior modifying pyscho-therapy. If she pushes herself into violent behavior while she's used to doing strong self-identification with the character, you can end up with that roleplaying expressing itself when they are out of character. I've seen it before. It's usually not a big deal, and usually if your friend can be gently reminded that they are being their character it goes away.
Ok, so that paranoid bit out of the way, you do this in one of two ways - just like you would if you were role playing in pyschotheraphy or else by teaching the person method acting. Or more likely because this isn't some formal course work and you are trying to have fun, by some ad hoc combination of both.
One of the first things you have to do which I don't think your friend has done is to imagine why this character is angry and why they want to lash out, intimidate, and hurt things. Then after carefully imagining why the character is angry, they have to build that anger in their mind and feed it - in exactly the way that you wouldn't feed a real anger. Your friend needs to in every situation in the game look for how she can relate what is going on to the source of her anger.
My advice is to give her some unambigious evil things to blow away not just because it will help her be angry, but because it will help her drop the anger or pack up her 'angry toy' when shes done with. That isn't always an easy thing to do, especially if you play with the toy regularly. Contrary to popular belief, an outlet like this for venting does not in fact make one more in control and less angry otherwise. She's fighting I would guess against her own reasonable psychological constraints for expressing anger (unless she's a person who is prone to being subservient and supressing her desires, in which case a little expressive training might be a good thing). In order to get around her minds defenses, she has to build this alternate persona that she is clearly labeling 'pretend' and then find a way to enjoy that anger release.
One way to do that is let her be the DM for a while. It's alot easier to do this when you don't have a strong identification with the character or with the character's success, and when you are switching roles all the time.
