Two things really - I agree with her therapist, and recommend that she is seen by a pediatric psychiatrist who can evaluate her and prescribe if needed.
Secondly, on the home front. Children of her age are rather liable to push things to see what happens, and what they really need are firm boundaries. Being ‘soft’ just makes you easier to manipulate, and anger just teaches them to be angry when they in turn are faced with a difficult situation.
Shee is quite old enough to know about actions and consequences. We humans only indulge in behaviour that bring reward of some kind. Only when that reward (whatever it might be) disappears, or the consequences of our behaviour promise to be unpleasant do we consider changing what we do.
Here is the clue to sorting things out. When you are faced with non-co-operation – give her choices, and make sure she understands the consequences of their choice – and always follow through. If you don’t she’ll just get confused. Please make sure though that all her small successes are praised and occasionally rewarded. Choices are not always about punishment - they can be offered in advance.
"We can go to the park today, but only if you promise not to shout at Jane. Now, would you like to do that or would you rather stay at home?"
Make it clear what you expect "When we go to see your best friend Judy today, I expect you to play together nicely. If that is not going to happen, may be we should just call and say we are not coming."
When it does come to punishment, try to make sure that it is something that will have an impact "To your room, and no TV today at all." And make sure you vary it otherwise it becomes stale, and therefore more or less ignored.
Ask her too, what she is prepared to do to change her behaviour in future – tell her to research what might help her what help she feels she needs, and even consider a ‘contract’ between you. In other words, involve her in her own change, with a prospect of a small reward for success.
Never, never be blaming or accusatory. Stick to facts, tell her how you feel about her behaviour, and make sure she understands that while you love him, her bad behaviour Is hurtful and will not be accepted.
I’m going to suggest that you get a copy of the book “How to talk so kids will listen, and how to listen so kids will talk”. Its ISBN is 1 85340 705 4.
Not only will it help you turn things around round it is also a good read!