Hi, OP -
This is a great question. The answer has become complicated. (I am a former Russell Group STEM admissions tutor.)
Fundamentally your DD’s instinct is correct: it is better to do the best you are capable of in three A levels than to slip in even one in order to do a fourth.
Some degree programmes, like certain programmes at Imperial, will make two offers and one will allow for slippage, but this is relatively rare. Oxford may well make a standard three A offer anyway, but that will only be to applicants who have met its other criteria and are usually predicted (significantly) higher grades.
We are seeing an informal trend towards more and more tiered offers, wherein the first tranche of offers go to a group who greatly exceed the standard entry requirements, then another tranche to a group somewhat less stellar, etc. This board has recently had a number of threads from parents of applicants shut out of programmes for which their PGs made them sound a good match. sometimes those programmes guess wrong, and must take underqualified applicants in August. The system is not working well. But I digress.
Some degree programmes that don’t require FM ‘strongly recommend’ it. Sometimes a subtext here is that if your school offers it, you should be taking it and it counts against you if you do not.
DD could start thinking about programmes that interest her. If she comes across this language, she can email the admissions team for the degree programme explaining her dilemma. The wording will need some thought - your paraphrasing of her dilemma is fine; she just needs to make sure she doesn’t sound the slightest bit like she is looking for an easy path (no reasonable person would take it that way, but one never knows). Of course you can help with the message but it should come from her.
If relevant, a few elite universities will say that Maths + FM + Physics, if taken, is ‘really’ just 2 or 2.5 A levels because of the mutually reinforcing nature of the subjects (if one elects Mechanics insofar as possible). I can see a good case for 2.5.
I realise it may sound like I am implicitly making a case for FM but unless it is recommended for the degree programme that is not my intention. Doing her best and learning thoroughly is most likely to be useful for DD.
@TeenToTwenties idea of starting FM and dropping it is good, but often conscientious YP become invested in the Sunk Cost Fallacy. Would DD find it hard to let go?