Winnie
I had this with my dd (15 today!) for months, coming to a head a couple of months ago - heartbreaking, isn't it? I think so much is invested in a 'best friend' of several years standing.
DD was being dropped when her best friend moved a short distance away and started walking home from school with someone seen as funnier, livelier, more interesting, more fashionable than dd: best friend could find no time for dd at all. Was going to go on holiday with new friend, doing everyhting with her. Worst came when on a Saturday dd had arranged for b/f to come round to have a heart to heart (all carefully planned, mother at work, father and brothers our for afternoon) when b/f rang up at the last moment to say that she had too much homework to do and couldn't come. DD devastated and rang me at work in tears: I raced home and took her out for the afternoon. On Sunday she rang up b/f again to see if they could talk, and was told that she was going running with the other girl, and DD could go too if she wanted. Said no, and did not talk again all day.
I actually did speak to b/f's mother, who felt it was a flash in the pan. And it has turned out that way in the end.
One thing dd did was to write a letter to b/f which she kept until it was pretty tattered before handing it over, basically saying what she really liked and valued about her, and explaining how unhappy she felt. Big courage to do this I think, as I could see all sorts of possible outcomes. But the actual outcome is they are back together again, although I think dd is beginning to be more independent which is probably good, and making other friends too - not easy when you are shy.
Was the split something your dd started, or the other girl? Much sympathy, this is a really horrid thing to happen.