Which A-level to drop - French or English Lit?

Insisting on 4 is pointless. She needs to work backwards from what she wants really, she must have a rough idea as this time next year she will be starting to look at unis. Both French and English Lit are difficult for different reasons and both have incredibly high grade boundaries.
If she doesn't mind either I'd first check what curriculum her chosen college does for English Lit and what books the teacher has chosen to be studied. That can make or break the enjoyment of the course. After that, I'd just choose whatever subject she's stronger in as they're both humanities and will keep similar doors open.

Why is music non-negotiable? It is the easiest to replicate outside of school - if she is playing at a decent level and wants to pursue it she will get UCAS points for grades 6 and above for a related course and audition will be what gets her the place not music A-level.

Firstly I would say it’s not a good idea to take four A levels; it’s a lot of work leading most likely to dilution of grades, and as such not advised. (FM is an exception).

You say she is taking maths to keep options open – but what options? As a sole science it is unlikely to open up STEM options (which is fine of course) and it won’t really open the best unis for maths as many of them look for FM (unless not offered at the school). Maths A level is a great one to have, for sure (my DS2 took it) but it’s not easy and taking it only “to keep options open” when it doesn’t is perhaps not the best plan IMHO. If she loves maths of course then happy days. What is her GCSE PG for maths? DS got an 8 and then a hard-won B at A level. An A was perhaps possible if he had really given it everything.

So maybe drop maths? If she is doing maths bc she loves it, then drop the one of the other two she likes least or is weakest in. Eng lit opens doors to Eng lit degrees, French to MFL degrees; dropping either will basically close that door (again, fine; all A level choices will close doors). So might she want to take MFL or Eng lit further? What is her PG for French GCSE? I would recommend at least a 7 to take A level.

MFL is my subject btw and I love that people want to take it further. You may read that native speakers mean she won’t stand a chance of a top grade but I think that’s not really an issue if she is prepared to work hard. The exam does not look for native speaker ability (and in fact states that).

So overall I don’t have an answer for you! Just some things to think about I guess. Ultimately it needs to be her decision and one she is happy with.