I don't agree. I know a number of girls who have gone through this stage and come out the other side as lovely people who are ashamed of their former behaviour. In fact, even when they are in the middle of their bitchy phase many of them are nice some of the time, as I'm sure their families would agree. Get them away from their audience and they can be downright human. Only trouble is, once they are back in the crowd again the bad behaviour returns, so it can be hard being friends with such a girl.
Sure, some girls have more of a tendency to behave this way than others and they may get stuck in their ways forever. But I think that very often it is just a response to the environment they are in. The worst environments are those that are quite insular, where people spend most of their time within a fixed social circle, such as schools and offices. In such a closed situation, falling out with the group is social suicide so people tend to play the game - whatever the game is. It's often bitchiness, backstabbing and social climbing.
I do, however, agree that a good antidote is to maintain friendships outside of the girl-classmates. This could include boys, children who are younger and older, and children who go to different schools. It doesn't fix the problem, because your daughter will still be stuck with these girls for many hours a day. But at least she will see light at the end of the tunnel, and will realise that not everyone behaves this way and it is not the absolute end of the world if her classmates turn on her. Having a good friend gives a boost to the self-confidence as the girl realises that she is likable and it may not be her own fault that other people are being nasty to her.
Here's a good book which may help give your daughter (and you) insight into the details of how it all works, and some ideas about ways to survive: "Queen Bees and Wannabes: Helping Your Daughter Survive Cliques, Gossip, Boyfriends, and Other Realities of Adolescence" by Rosalind Wiseman.