I don't think her GCSE grades are the main issue. They're good enough to give it a go if its what she wants.
The issues are:
(a) Does she want the intensity of Oxford?
(b) Does she want the Oxford course, which as already noted elsewhere differs from the type of MFL course at other unis in terms of literature focus?
(c) Does she want a university with a college system (Oxbridge/Durham)? It's a different environment.
(d) Does she like the vibe of the city/campus itself? Can she see herself living there? Does the university meet other criteria (extra-curriculars, particular sports team etc)?
(e) What does she want out of her year abroad as part of MFL degree in terms of location, how the time is spent and the whole set up? For example, some are all studying at an overseas institution, some you can choose some time not studying e.g. in a work placement. Some you live in university halls where it's all pretty much arranged for you and others you have to arrange private accommodation in a foreign country in a foreign language and negotiate everything that goes with that (landlords etc). Some courses what you do on your year abroad counts towards your final degree grade and others it doesn't.
Where geographically does she want to go on that year abroad? This differs massively between courses: It's the difference between being in Paris or the back end of beyond of rural France, or between being in Barcelona or Buenos Aires. Some you have to spend the whole year in one place, others you can move (especially if doing mixed MFL - you can do part in Spain and part in Italy, for example). These elements also impact on how much that year is going to cost financially and what the experience will be like (and how safe it is/how easy it is for Mum to visit or her to come home for visits/how easy it is for Mum to bail her out of problems if necessary). It's really worth spending some time and energy thinking about and researching this part. For MFL I would have thought this would likely be a very influential if not decisive factor in where she choses to apply.
(e) Does she want to be committed to her current MFL, or does she want the opportunity to pick up new languages ab initio, which is offered on some courses?