Everyone has offered great ideas above. I'm a 'yes' for Further Maths also if Oxbridge is involved and, even more importantly, if she would enjoy it! Why not review the syllabuses online for each - Maths and Further Maths? They are easily found via Google. Which is more stimulating?
NB it's not just what Universities list publicly as requirements that needs to guide students (in relation to A levels) - but also what their competitors/peers will have - for Oxbridge, many will have plumped for Further Maths as it is well respected. I'd ask Admissions what percentage of students had Further Maths that applied last year for any key universities you are interested in.
Please also see the following which advocates Computer Science ...depends on which area she is interested in, clearly! Essential =Maths, Physics but also valuable = Further Mathematics, Chemistry, Computing/Computer Science ...see
russellgroup.ac.uk/media/5686/informed-choices-2018-1-6th-edition-final.pdf
I'd ask her to perhaps consider her future careers thoughts a little more closely...(any research and time she spends doing this will always yield fruit!)....
To look at Physics linked careers more closely - see www.prospects.ac.uk and go to 'careers advice' then 'what can I do with my degree' and select Physics and other STEM degrees - which roles seem interesting? Which companies offer roles in these fields?
Then go to Linked In and look up current professionals in these roles - which degree did they do (placement year? Where? Which country did they study in that had money (especially post Brexit) for this area of research- US or elsewhere?).
I'd then look at work experience - which organisations would be her top ten (based on jobs she's interested in- where are they based geographically - can she reach any easily)? If yes, could she gain some work experience over the summer holidays with them? This could help her to identify areas of interest career-wise also.
Perhaps consider gaining a STEM mentor - physics linked obviously- there are hundreds of STEM experts that give time to mentor - she'll need to understand which overall areas of physics interest her and then find someone (through LI or via Google) whom will help her to explore areas and perhaps open work experience or internship doors for her.
Doing this research and work experience might help her to fine tune A levels but more importantly, it will help her to get excited about ideas for the future, which helps motivate during hard study!
Good luck!