Unless she's on the verge of a more serious actual illness (mental or physical), i wonder if modelling her a bit more of resilience and a stiff upper lip might help? sort of jolly her through a bit?
As other posters have said, this is normal. And she needs to learn strategies to deal with it. Can you be loving, sympathetic, but don't collude with her over her panic?
There IS far more to life than a First or a high 2, i. If she doesn't get those results, it is not a disaster, even for further postgrad study.
I think it's really hard particularly for young women - we live in a sexist patriarchy where all young women get the subliminal (and often quite overt) message that they are not really fully deserving of their existence unless they are exceptional at everything. They have to be clever, friendly, pretty, slim, well-dressed attractive[to boys] and so on ...
A bit of a bracing "Of COURSE you can do it. Now go out there and SMASH it!" might help at this point more than soothing sympathy. Maybe?