We didn't have too many problems until about age 10+. From then on it got decidedly tricky.
I have always tried to impress upon my dc (well dd's mainly, ds seems less complicated re friendships so far) that at school it's better to be generally friendly with a lot of people and to buzz from group to group rather than to be too exclusive.
However this advice is only ever followed minimally - usually following some sort of fallout - because girls seem to be hard wired into wanting to be exclusive and will gravitate to certain disaster in this no matter what you say.
Always things reach a point where they couldn't be any closer if they tried and everything becomes too intense. At that point someone in the group inevitably detonates the thing with some minor misdeed or action and because it's usually just a fragile house of cards anyway the whole thing collapses. They fall out and things cool off.
I think barring outright bullying, you can only be there to offer cuddles, chocolate and encourage the next friends. It's very wearing though.
The trick is not to get too involved. As dh says. Be there to support her but don't help her dig the hole and jump in with her over it. I try to bear that in mind because there have been times when I think I probably have taken it all on board too much for my own good especially in yr9 where things can get quite nasty. I've dreaded dd coming coming home some evenings to hear the next installment. I feel so helpless and hurt for her at the same time; she is still my little girl after all.
Although I have noticed sometimes I seem to worry about it more than she does which goes to show that really they need to resolve it in their own way and in fact often can resolve it which is empowering and helps them learn how to deal with such things.