OP - it's a shame that the school doesn't offer German A-level. But if she wants to read German at university she could make a good start by doing some self-study this summer, so she doesn't forget what she learned at GCSE.
She could also watch all of Heimat! And some major German novels in translation. There's plenty of time for relaxing too. But if she wants to apply, she should treat the summer as an opportunity to read widely and get ahead.
Some of my cleverer friends took extra A-levels under their own steam because they found the material enjoyable and easy - and didn't put themselves under great pressure to do well. The fact that they 'don't count' doesn't mean that they don't count intellectually! She should be willing to work around the standard expectations and school models, to do what is good for her, and stretches her.
On going home at weekends: I do remember that weekends weren't necessarily as busy as weeks in terms of socialising. Student club nights were during the week, college discos were usually Friday nights. There was a good deal of sport on Saturdays. That's a long time ago, of course. Either way, I don't think it's a problem if she goes home regularly, though better not to have a fixed plan in advance and rule herself out of getting involved.
Final point in response to the PP who said that admissions tutors don't necessarily know what they're doing: well, it's like any interview panel. There are so many very able young people, all with outstanding grades. The panels do their best. I've heard that there can be some horse-trading between tutors with very different views over candidates for dual/triple honours degrees. I've also heard of a case where a tutor realised a student was brilliant, but that they would find them difficult to teach, so recommended them to another college. That student probably assumed they were borderline, which they were not.
Then the students turn up nearly a year or two later - and may have changed their mind in the meantime about how committed they are, which doesn't mean the panel got it wrong at the time. Or, they may decide that university is for coasting, while school and the workplace are for hard work. Again, it doesn't mean the decision was wrong.
Or - the panel may well make mistakes, just as we do when interviewing for employment. It's a noisy process.