It has been a while since Ds was also rejected from Cambridge after interview but statistically I think he was a 1 in 5 chance at application stage, then 1 in 3 chance at interview meaning that a shit load of them get rejected, Ds had 4 A star prediction and achieved 4 A stars. Statistically the odds are stacked against every applicant, not just for Oxbridge but for many other unis too and for some courses the odds are even higher, ie Econ at LSE where 70% of their intake are international students.
STEP which is the entrance exam for Cambridge maths is also manipulated every year by Cambridge to cull (I believe) a percentage of the maths applicants who will probably get 3 A stars or higher so meet their entry grade requirement. An offer from a university is a legal and binding contract, if they have 300 places and have offered a place to say 340 students they need reduce that number down. Most will achieve their entry grades but then STEP is used to amend the passing level to reduce it down to the 300 students.
Warwick is an incredible university. Incredible. It is part of COWI (Cambridge, Oxford, Warwick, Imperial) and lots of students would give their right arm to go there.
There are lots of redirected from Oxbridge threads on the Higher Education board, and there is life after the rejection. There are also threads on current Oxbridge students and it is intense, so if she is feeling this low now maybe Cambridge wouldn't have done her mental health any good. The pressure is immense.
For a maths degree they really do not like a gap year and if she applied and was rejected again then what? I think she should look at her other offers and go from there. Maybe visit the place again before results day.