Is there a reason for Oxford and not Cambridge, given she could do philosophy by itself there?
If it has to be Oxford, I agree with @Hoghgyni that deciding which joint course she wants to apply for should be the main focus of her reading and research at the moment.
How is her maths? While not being required for PPE at Oxford, it is recommended, and the Oxford website has links to stats showing iirc that over 90% of successful applicants have A level maths. So if PPE is the most likely course for her I guess she should look at what's required in the aptitude tests, do some economics reading and maybe have a look at formal philosophical logic, which is a bit different from anything in A level philosophy and definitely more 'mathsy'.
Which of the other joint courses has she considered? Which are possible with her A levels? If she's doing philosophy of religion as part of her A level, is she interested enough in broader religious topics to consider philosophy and theology?
Otherwise I'm not sure which courses are open to her: the psychology, philosophy and linguistics one needs a science or maths, I believe. There's Classics ii but having a particular aptitude and enthusiasm for languages is presumably crucial there.
There are plenty of philosophy reading lists available and I think the AQA syllabus has a recommended reading section with extracts from primary texts. She could also look at /have a go at the philosophy aptitude tests. But unlike a majority of philosophy applicants she already knows she enjoys and is good at philosophy, whereas choosing a course at Oxford and maximising her chances of being seen as a strong candidate for the other subject(s) involved could be more challenging.