L.P.
Hi B.,
I would say that you need to nip this in the bud right away. Not only is this hurtful to your daughter's friend but also to your daughter.
It sounds like your daughter is embarrassed to be friends with this girl at school so this is more of a problem dealing with her other friends than it is dealing with this specific girl. Your daughter is probably trying to fit in with the more popular girls and is following their lead and for some reason they decided this other girl wasn't good enough for them.
This can lead to more problems that you probably would not want to face as a mom down the road if your daughter doesn't learn that she needs to love herself and be herself no matter what the "popular" girls do.
If this is the case,I would sit her down and ask her about her other friends and explain to her that she is a wonderful person and does need to do things to just fit in.
I could be way off and I am sorry if I said anything that might offend you. I don't know your daughter or the whole situation. I am only seeing it by what you wrote.
I commend you for trying to teach your daughter good morals. It is very hard to do today with all the worldly influences out there.
I hope this helped in some way,
L.