I went to Cambridge for my undergrad and am now a researcher at Oxford (humanities but not languages). Firstly, congrats to your DD, they’re some excellent grades! She should be very proud of them, and they put her in a really good place for applying to her choice of unis.
I went to a very good state school which sent a handful of students to Oxbridge each year, I got very similar grades to your DD. I really think she should apply to Oxford if she would like to go. Her GCSE grades are only part of her application - they’re considered alongside her personal statement and super-curriculars, entrance exam (if there is one) and interview (if she gets to that stage). From my experience, they’re a perfectly normal set of grades for an Oxbridge undergrad to have.
Oxbridge don’t care about extra-curriculars (i.e DofE, sports, music) if they’re unrelated to your course. Yes you should have a small paragraph on your personal statement about them, to show you have skills other than being good academically, but just doing your normal weekly after-school clubs or learning an instrument at any level and writing about how this taught you teamwork, organisation, resilience, perseverance etc. is perfectly fine.
What Oxbridge do care about is super-curriculars. These are things you’ve done related to your course. If your DD writes strongly about a range of relevant super-curriculars she has done in her personal statement, showing her passion for her subject and how she’s engaged with it above and beyond her school classes, she’ll have a strong application paired with her GCSE grades. If her super-curricular offering is rather lukewarm, combined with her GCSE grades, she will be amongst a pool of far more convincing applicants.
She can’t do anything about her GCSE grades now other than apply and hope for the best, so there’s no point in worrying about them (although easier to say than do obviously). What she can do over the summer is develop her super-curriculars. A non-exhaustive list of (free!) ones which, when written about well, Oxford would be interested in is: critical review comments on books she’s read / films & documentaries she’s watched in her target language, attending public university lectures (in person or online) about topics related to her course, writing about aspects of online courses she’s taken (look at MOOC courses), volunteering work relating to translation or supporting people speaking her target languages, helping out at school language lessons or language clubs. If you have the finances to pay for language schools, visits to countries which speak those languages etc. these are also options but not necessary.
If your DD shows passion for these, with her GCSE grades, I think she will be a strong applicant to Oxford. Wishing her the best of luck and I hope she applies!