@Neolara I work in outreach for Cambridge Maths 
I would say Very Good rather than total genius!
A couple of things she could do to help her decide:
Explore the Oxford Online Maths Club. James is great, and the live sessions are interactive via slido.
Start working through the foundation modules from the STEP support programme. The foundation assignments are designed for use in year 12 and the first few technically only need GCSE maths (they are still quite hard though). I think it's too early to look at STEP papers as even STEP 1 uses all of A level single maths and she probably hasn't covered the material yet.
She might be interested in the Maths makes a difference webinar materials - this was a collaboration between the outreach teams at Oxford and Cambridge (we can get along when we try!).
Other things to consider are the various admissions tests. Quite a few universities will accept a good result in TMUA for a slightly less good A level result in one of your A levels. They will also accept a good MAT or STEP score as well.
Oxford and Imperial require students to sit the MAT.
Cambridge, and Imperial if you missed the MAT deadline, require you to sit STEP
MAT and TMUA are sat in October/November and the deadline is quite early in year 13, so if she wants to sit one of these you need to watch the dates! These both basically rely on AS level single maths (with perhaps a couple of small additions and TMUA has some stuff on proof and logic).
STEP is sat in June of year 13. STEP 2 involves A level maths and AS FM, STEP 3 the whole of A level Maths and FM (STEP 1 is no more
).
If there is anything I can help with, please do @ me or DM me! (I don't check this forum that often...)