Duolingo is a great start and although sometimes tedious, will give a solid grounding in a not particularly aware you're getting a solid grounding kind of way! She can play with friends which can be motivating - it's free so get her started on that.
For vocab look at quizlet and memrise. They both work in the same way - you will find lots of teachers have inputted tons of vocab for GCSE and A Level. Just a matter of personal preference which one she prefers.
School may have a subscription to linguascope so use that if that's the case. The 'beginner' is just words (but words she will need at GCSE) and then the intermediate is a bit more complex and more geared towards GCSE (although last time I looked, hadn't been updated to the new specifications). All good stuff, if repetitive and predicatable.
languagesonline has a free part and a subscription part. Just use the free part - will help consolidate words if nothing else.
There is fun stuff such as lyricstraining.com which helps tune in to the language for listening purposes - don't necessarily need to understand, just find a song she likes that's on there and listen and listen.
If she is interested in reading, look at the French Cosmopolitan or Marie Clare or similar. She can help herself read this more easily by running the individual webpages through lingro.com - what this does is make all the words clickable so when you click, you get a translation. It takes some of the frustration out of trying to read stuff which is harder than your ability.
If you go to amazon.fr and look for Kindle books, she can probably find free children's books which are worth a read - pictures make it a bit easier to understand. Focus on what she does know, not what she doesn't (which is half the problem - they freeze with lots of content and go into 'can't do it' mode rather than try and piece together a story using what they do know and some logic).
If you have a netflix account, there is plenty of French content on there. Just use the search facility. Just be aware that a lot of it is 15 plus. Some films originally in English (not all, unfortunately) can change the language to French and add English subtitles so she can watch her favourite films again and again and again....after a while, maybe change the subtitles to French which gives a feel of immersion but if you know the film, you start to recognise words and phrases. Ditto Amazon Prime.
More often than not, it's a mindset of 'can't do it'. The chances are other students in the class don't have a clue either. I would tell her to approach the teacher for help because if teacher is sending emails home, she is concerned and should be open to discussion.