How horrible. I'd start by acknowledging that it is healthy and normal to be very upset by this sort of malicious behaviour, and that in her position you'd feel this bad, any one would. But once she's been made to feel like she has the right to be sad, gently ask her a few questions: does she think any half decent person would behave that way? Get her to focus on her revised opinion of that girl, not that girl's opinion of her. And the same goes for the entire group. What is her assessment of them now? How would she expect them to behave towards her before she'd consider rating any of them highly enough to be trusted as a friend?
Get her to draw up a list of criteria in her mind for what a good friend is and if any of those girls try to isolate her or bully her again, get her to give them a mental score from zero (or minus) to ten on how worthy they are of her attention and reaction, judging by their behaviour. Like a mental bingo card of 'How likeable are you, bitch girl?' It puts the power back with her.
She needs a strategy for when she sees them next. The best one is to treat them as though they are invisible. If they force a reaction, get her to think of the score card in her mind and just to say, 'For someone who doesn't like me you seem very keen on spending time in my face.' Or 'We don't like each other, remember? So why are you desperate for my attention right now?' Or, 'Why on earth do you think your opinion of me matters to me? I only value the opinion of friends I like and respect.'
She needs to really steel herself to show their opinion means nothing to her. Less than nothing. If their lies are really malicious, then you may need to involve a teacher. And if their bullying techniques escalate, she needs to keep track. Save any horrible texts or online evidence etc.
Ask her about some people at school who she either likes and gets on with, or doesn't really know that well, but who also seem quite quiet or on the fringes. Those people might welcome a new friend. Suggest she gets in touch with some of them. It would be great to be able to walk into school with them tomorrow, sit with them or have lunch with them. If you know any nice girls who would do that to give her some moral support, you could suggest it.
When DS was having a hard time with friendships I asked him about a boy he'd hung out with lower down the school but they'd been put in different sets for everything and stopped socialising. I suggested they got in touch. It turned out that boy had been really struggling with friendships. They met up a few times and now they are both part of a new and very nice friendships group.