As a family counselor, the creator of a children's character building program and the author of Stop Bullying Now, I can hopefully give you a few pointers.  
You need to ask her to tell you what the problem is.  Kids will often tell you and if she doesn't, I often say what I think a child is feeling, but put it in the first person.
If this were happening to me, this is how I would feel.  That often opens the conversation. You daughter is being bullied and there are several resources on my website that might help.  Since one of the girl's is her best friend, I would have a conversation with the two of them and maybe even invite the other girl over for a talk.  BUT your leverage is the best friend.  
In the meantime, your daughter needs to not give away her power.  Tough at that age, but a lesson we all must learn. I have a CD on Self Confidence that would help and a book on Winning! that would give her some inspiration.  
Never allow others to define you.  Encourage her to have the courage to confront the person who doesn't like her.  If it continues, I would even have a pow wow perhaps with the other girl's parents.  As you can see, I am not want to sit back and let things grow.  At some point, it may be time to just make new friends and let go of the old.  That age is challenging for girls!!
Don't ignore it, but look at what can be learned.