Man do I hate this kind of stuff.
This will actually be up to your daughter to stop listening to the catty b.s. that the other cheer squad members are perpetuating. If they start talking about others when she is present, she needs to firmly excuse herself and go find something else to do.
She can cheer with her, but limit her interaction with her as long as she is being ugly and vindictive toward others. She has to know that she is doing this to her behind her back as well, so allowing it to go on in her presence isn't a good idea. She may not even have to confront her, just remove herself from the girls vicinity. Eventually this girl will ask why she doesn't want to hang out, sit with her, stay in the same room. Your daughter can then tell her, I don't like the way you act toward other people, I don't like how you treat them/me. I don't want to be around it. If you want to hang out with me, you cant say things like that. Period.
Then stick to it.
It will be hard, but your daughter has other friends.
I did this my Junior year when my best friend started hanging out with a girl that just thought it was funny to degrade and humiliate people in public(for laughs). I told her, "I don't like the way she treats people, I won't be around her and if you want to hang out with me, she will not be with you." I then proceeded to spend the entire summer with other friends. That next school year, my best friend told me that it really hurt her feelings when I wouldn't hang out with her. I said I was sorry, and I missed her. But I didn't want to be around the other girl. She admitted I was right and she gradually stopped hanging out the the catty girl.
do you want to hear the hysterical part of all this?
The catty girl caught up with me YEARS later when we were out of school. She told me she had spent most of that year being afraid of me, because I had told her at one point when she was being mean to someone that if she didn't stop it I was going to jerk her arm off and beat her with it. I told her I meant it at the time and I thought she had acted like a jerk. She didn't know why she ever behaved like that and was really embarrassed at her behavior. Do you know she's one of the nicest people I know now?
People who have to run others down to make themselves feel better are really scared, and don't like themselves. That doesn't make them any less awful, nonetheless.