OP, is she asking for your advice? Or just telling you what has happened? This is incredibly common and I don't think you can fix it for her. If you did, I think you would prevent her from learning an invaluable life lesson. I am sure we have all been an Emily, Fiona or Anna in the past. It's how you learn.
This is a very serious situation in her world but I do worry that by getting so involved you have given her the impression that she is actually in a very serious situation having done something terrible. She isn't and hasn't.
The best thing you can do is nod, say 'oh yes?' and generally encourage her to come up with her own solution which won't be the same as yours. If she says you don't understand, she's right. Maturity and hindsight make this situation inaccessible to you, sadly, and she will benefit hugely from you acknowledging that this has occurred, that she might not like the consequences but that it too shall pass, and there is life beyond school friendships.
If you were able to fix this for her by drafting a letter of apology and coaching her to take back what she said, she might still be doing this in her twenties. By all means offer some emotional support but anything above and beyond that will rob her of this vital life lesson that it would appear they all need to learn the hard way, either by doing it themselves or by watching it play out.
Tempting as it is to try to swoop in and fix their mistakes, you can't. This is hard for parents. Nobody likes to watch their kid upset but they often get over it and sort it out remarkably quickly, leaving the mother still thinking about it a week later when it is old news to them. I say mother because I have yet to meet a father who listened to the end of the complex he said she said sorry tale but apologies if dads do this. My DH has a very short attention span for this and he is probably right not to give it too much thought. I try to learn from his wise ways now we are on teen girl number 2.