Ah well, I can trot out that hoary old saying then, "It's their age....."
Unfortunately most girls have to go through this phase, it genuinely is all part of the learning how to get on with each other process.
It is also genuinely best if the mums involved can just smile ruefully, say "girls, who'd have 'em?" and let them sort it out (unless there is real bullying or cruelty).
They will have different best friends every day although some will have more domineering personalities than others and try to control others (so, a mini-version of adult life).
Not much help at the time you're going through it except to keep boosting your own child's confidence.