A level Germam (my subject! hurrah!) is a great spec. I know the AQA spec best (is that the one she is looking at) and I rate it highly.
No it is not all native speakers. And it is more than possible to get a top mark withouth being a native speaker. A friend of mine had a candidate this year who took Fr and German, not a native, not even staggeringly brilliant, just worked really hard and got 2 x A star. Not saying your DC will obvs but it is possible.
The most likely grade really depends but I would say on balance a B is more probably and an A is certainly possible tho would need strong commitment.
What was the issue with her writing? Is her grammar and vern knowledge secure? If not, looking at that and the basic vocab from GCSE before September would be beneficial.
It's a great spec as I say but it is a big step up. No more my hobbies or where I went on holiday. The topics are social and political – things like the changing family in German-speaking countries, the fall of the Berlin wall, racism, immigration, art and architecture, German music and film, politics and young people.
The big paper is language, reading and listening, translation into German and summaries in German. Then there is speaking which is a decent chunk of marks, with a research project on a topic related to Germany and then questions on a topic card. Then there is the writing paper, two essays in German on a book and a film. That's worth only 20% of the marks but is often a big focus (understandably). The books are good, I like Der Besuch, and the films are great (Leben des Anderen, Goodby Lenin, Lola Rennt).
The project for the speaking can be many things btw – in recent years (I examine this bit of the spec) I have had students talk about the rise of the AfD, Berlin after the fall of the wall, a specific German band, novel or film and more.
HTH but feel free to ask anything else :)