My personal instinct as a parent would be to catch this girl when no-one was looking and give her the fright of her life. However, probably not the reasoned reaction, and would probably end up in court! I think speaking to the other parent again would be the best answer, but also, try pulling one of the other girls that your daughter gets on with away from the crowd, invite her round to play etc, so that your daughter has someone to play with and some defense. I don't think I could stomache inviting the nbully girl round, not without my fingers clamping round her throat!
A boy my son played with from time to time, took his comic off him after breakfast club and threw it in a puddle. When i saw him the next couple of times, I just gave him a really dirty look. A month or two later, he came to the door for my son and I told him that when he came back with the money to replace the comic and an apology he was welcome to come in. I have never seen him since, and he stays away from my son completely now.
I think the bitchiness thing is a girl problem, boys tend to hit out and then are best of mates again - there is not so much of the emotional bullying.
Are you sure that your daughter is fitting in with the girls? I know that we shouldn't all try to conform for the sake of it, but at that age, it is important to have the right clothes, hair style etc otherwise you become a target.
Do you know what it was that the other girl said?