...also think she’d feel as though she’d failed if she went to a non-RG university...
That's definitely one notion that needs knocking firmly on the head. It might cheer her up to tell her that if the 'RG' were to be hatching their plans in the same place today, they'd be calling themselves the Kimpton Fitzroy London Group.
Which other universities did she originally apply to and get offers from, and why did the others appear less desirable at the time than this one when she came to firming / insuring? Going back a step further, which additional other universities did she initially consider and why didn't they make it into her UCAS form? Looking at all of these de novo, which would she apply to if doing UCAS for the first time today, and, assuming offers from all, which would she firm? (Of course, this might not help if she was persuaded from the outset only to look at RG universities, only to list the 5 'most prestigious' on her application and firm the <ditto>).
That being said, like pp, I would seriously advise her to request to suspend her studies for a year instead of dropping out, take stock of her situation in a less pressured manner, and look at submitting a whole new UCAS application within the normal time frame (including autumn open days etc) for a 2025 start elsewhere, if that's what she decides is really best for her.