If she is keener on science then a grade 6/7 is great and all she will ever need tbh. In fact she won't need it (as in no uni requires an MFL) - but what I mean is, unless she is applying to Oxford or looking at vet med, a 6 is fine. And a 7 will tick the Oxford/vet med box.
Still tho – on the vocab – on the AQA website it's sorted into some key words (time phrases, opinions, key verbs) then listed by the 12 sub-themes. So what you could do (if you really wanted) is print out one of those lists and tick off all you know; then set yourself to learn (say) 10 of the unknown words each week (or five each day, or whatever seems reasonable).
I should stress that there is no need for a candidate to know every word on the vocab list. It's not intended for that really. Completely possible to gain a 9 at GCSE without knowing the word for (glances at random) pamphlet (el folleto, fact fans). So she shouldn't worry about not knowing words. If there are words you don't know in the translation, the best thing is, if translating to English, work out what kind of word it is (adjective, noun) and see what would make sense; if translating to Spanish, say something similar or write round it if possible – sometimes those answers are also acceptable.
Btw you mention Higher only words – you do realise that for Higher paper all or any of the words may come up? The Higher only words are only relevant if you are taking Foundation – as in, you don't need to learn them.
One of the best things to do is practice papers. There are only so many actual past papers but papers from other boards are good for practice as well (as long as she realises the structure of the actual exam may be different. And there are loads of practice writing questions online. Her teacher should be happy to mark anything she does. I know I would be (if I were her teacher I mean!).