Before thinking about contacting unis, writing about extenuating circumstances etc, you simply need to speak to school.
As you say, she only got the predictions yesterday. It’s not a crisis yet. Many schools and colleges expect to alter some grades. They expect to hear back from parents and to be questioned about grades. Some will change and some won’t.
Whwn she’s calmed down, speak to her more and dig down into her performance across the year in BOTH Maths and FM. Most will have a lower FM prediction. If you think it really is unfair, gather evidence of marked papers and prepare for a September meeting. Rememeber , that the unis she likes need a higher grades is t a good argument. You have to choose the unis based on the grades. You should only make a case based on attainment and potential.
Finally, have a think about if Maths is definitely the thing. Maths at good unis is vv hard. The school’s initial thought is a B for FM. That might not be correct, but their initial and immediate thought isn’t that she’s a whizz mathematicians who will get A at FM. That’s worth considering, because Marhs more than anything else, can be miserable and soul destroying if it is beyond you. This could be their error and actually she is A or A standard for FM, it if she really is a B, then it might be worth considering if this is what she really wants. And I know people will say ‘But B is a great grade’. Well, yes it dis is a good grade, but for a Maths degree, you really do need to be managing top grades at A Level without huge difficulty, so the step up at degree level isn’t impossible. People have their Maths limit and if yours is A Level, degree Maths won’t be fun.
But to all those saying write to the unis for extenuating circumstances…..that’s all very premature. There’s time for predicted grades to be adjusted if appropriate. Digging down into it all more with her is the first step, discussing degree choices etc. Then it might be worth considering tasting schools no going in for chat. Plenty of stages still to go before the UCAS form is sent off. It feels urgent when they’re upset and very dramatic, but you have to take a deep breath and there will be a way through.