My mom used to tell us, "if you cannot play nice with each other, I will not let you play together at all.
My mom never made a false threat in her life, so I knew to either cut it out, or quit hanging out with these girls.
In reality, they need to figure it out. You can say, "I do not want to hear about this any more. If you want my help ask."
Or , "if you need some suggestions ask or else, quit complaining about each other. "
Your daughter is old enough to hear true examples from your end life.
Do you have a best friend? Do you have a few really good friends? Friends for / from different parts f your life? Really good neighbor friend, then the college best friend, your church buddy friend, your work out friend, your at work friend, your husband? And yet you can all get together and all get along. You can visit with one and no one gets upset...
Explain th to your daughter and remind her, she does not own anyone's friendships, and no one owns hers.
It is better for all of them to try to get along, because some day, she maybe the odd one out, and that is not a good feeling.