(Haven't read all the responses.)
Ugh. My dd went through that in 7th grade too. We worked hard to teach our kids to be loyal and faithful to their friends, so it came as a real shock when only a couple weeks earlier her "best friend" wanted to talk to her about her upcoming birthday party. They were planning it together, and then my dd wondered why she hadn't gotten an invite yet. She was riding her bike one day and happened to see several girls walking to her friend's house with birthday presents in hand. Yep, best friend had decided she wanted to invite her "new" group of friends. I was just as mad at her mother, because really, wouldn't her mother (also a friend of mine!) step in and tell her dd to include her best friend of the last couple of years who had planned the party with her?
Anyway, it was a real eye-opener for my dd. The first thing that helped was my voicing the thoughts that I knew my dd wanted to say, but thought she shouldn't. It was good to confirm her feelings and let her know that they were fine and completely normal!! Then we talked about how in reality, it was not something personal against my dd, it was more just a weird, very unkind decision that her friend made. Then we talked about how when you do find a good friend who is loyal, she's a real treasure. And then, we went out for ice cream.
And yes, later I called my friend (the mother) because I was so flabbergasted by what happened (and because she was a friend of mine). She seemed to feel really awful about it, and then her dd called my dd up a couple days later and invited her to a special weekend sleepover, just the two of them. My dd didn't feel comfortable going of course, and the friendship never really resumed.
It's a tough lesson. Shower her with love!!