There is a simple answer. Nothing. There is honestly nothing a parent can do to make this better. Best friends on Monday, enemies on Tuesday, best friends again by Friday. There is no rhyme or reason for this fickleness. Kids in middle school are especially susceptible to this jockeying for friends. They are in the midst of going to what I call the "buffet of friends." In elementary school, friends are often chosen by default. Perhaps your best friend has kids the same age, and by default your kids become "best friends." Or maybe your neighborhood is full of kids the same age, and since kindergarten they have been hanging at the bus stop together, taking the bus together, and getting off the bus together and by default end up at each others' house after school, so easy. Think of this like taking your kids to a Chinese buffet. When they are young and overwhelmed by the options, you make their plate up with those things they will eat, chicken wings, fried rice and spare ribs. Now as they get older, they go up to the buffet themselves and are astounded and excited about all the choices, and are anxious to give them a try. Choosing friends in middle school and again in 9th and 10th grade is like going to the buffet for the first time. Wow, look at all these options. I think I would like to try this friend, or that friend.
This means that some kids will do the leaving, and some kids will be left behind. Now that these teen brains are working on overtime, they are thinking more deeply about who these people are they call friends. Whereas in elementary school they only need a warm body for "playing", now they look for friends to talk to, and to share common interests with. They are less interested in what you have to play with and more with what do you have to offer me? Do I like your personality? Are you too quiet, too loud, to bossy too pretty, not pretty enough? etc etc. Are you fun, do we like to do the same things together? Often in middle school and then again in 9th grade, some kids are ready to transition to more teenagery like behaviors, partying, experimentation with the opposite sex, drugs and alcohol, while some kids are happy with less riskyish behavior.
All this is a set up for feelings of betrayal and exclusion. It is painful, and the good news, is they will get over it. As for your role, there is not much more to do than understanding their pain, and providing tons of TLC. If you insinuate yourself into these friend dynamics you will regret it. Perhaps you have never liked the girl who has just defriended your daughter, and you tell her so. Thinking you are making it better, you wax on and on about what a bad friend this girl has been, and good bye to bad rubbish! The only problem with this is that the next day, when the girls have made up, your daughter now knows you hate this kid, and will never talk to you again about her.
I talked to a mom about this yesterday at one of my "Ask The Expert" parties whose daughter was experiencing all these friend complications. She said that her daughter would come to her crying and in her effort to make her feel better would try to solve the problem for her, by giving her all kinds of strategies. The daughter, not looking for help, just a shoulder to cry on, then gets angry at mom for interfering. Thats' what I am saying. Stay out of it!!!!! Your kids need to learn to figure this all out for themselves. Obviously if it is more of a bullying situation, it may require a different strategy, but if it is old-fashioned cat-fighting, just let it be. Your kids will have a lifetime of friendships for which they are now in training. It's a bit like basic training. In the beginning, you never think you'll get through it, and then you get stronger and smarter, and you get better at figuring it all out. Just be patient, they'll have to sweat a little.
Now having just told you to mind your own business, I do have one caveat. A parent recently told me of this situation. Her daughter went to a friend's house with 7 other girls for a weekend night "girl party." It seems that this girl cherry picked 5 of the girls to sleep over and left the other two out of the sleepover. As you can imagine, those two girls felt like s**t. It didn't seem like the host's mom had any idea this had happened. If your kid has a group of friends over, there should be a proportion rule. In the example above, the parents should have been aware of the situation from the beginning, knowing who was invited for the sleepover. In a large group of 7, I get that all the girls sleeping over might have been too much, but 5 out of 7 is just too exclusionary. Have a rule in your home about sleepovers that states, either everyone or just 1. It would make sense the host girl wanted someone with her to finish out her fun night with the girls. I think all the girls could have understood the one rule, but they didn't understand the 5 and not you two rule. Your teens might need some of this kind of help. You won't choose the sleepover friend, but you can teach them about inclusion!