For finance FM isn't necessary for any "straight" finance courses I'm aware of, and I expect it'd be overkill for the vast majority of such courses. Really outside of "financial mathematics/statistics" or actuarial science type courses I think FM is probably not especially beneficial in that realm.
For economics FM is preferred or implicitly required by a very small number of universities (basically Cambridge and LSE plus maybe UCL and Warwick). It's very useful for any economics degree but one wouldn't be disadvantaged to apply without it except those unis (and even among those, Warwick does in fact state that it's not required and applicants without it wouldn't be prejudiced for not having it, although the Warwick course is especially mathematical it's worth noting). Notably Oxford does not to my knowledge have any special preference for it either (albeit I think it's still reasonably common for history & economics and economics & management applicants to be taking it; PPE applicants it's less common I believe).
Note though LSE specifically do indicate for single honours economics (and the joint honours variants with maths) that they expect FM to be taken if offered. For joint honours courses with other subjects (e.g. philosophy, politics, economic history) I don't believe they have the same expectation. They will expect the referee to explain that the school does not offer it if the applicant isn't taking it and indicate that applicants without it would be less competitive (and given something like 95% or more of successful applicants in most years have it I suspect it would render one non-competitive for that course to have it offered but not take it).
Equally though worth considering - if he's not enthused by FM then he may find those very mathematical economics courses less appealing anyway. Also it's important to keep in mind economics as a degree is not a "training course" for finance - it's an academic field in its own right and if his main interest is finance rather than economics itself, then it'd probably be better to aim for a finance degree anyway (equally, one doesn't need either an economics nor a finance degree to work in finance as a graduate anyway, so if he actually really loves Egyptian hieroglyphs he can certainly go study Egyptology at e.g. Oxford then become an investment banker!).
Outisde of that and the requisite ("regular") mathematics (which is required for the vast majority of economics degrees in the UK), his other subjects are really up to him. A-level Economics is not required by any economics degree I'm aware of, and biology and French are equally acceptable for those or really any other course that doesn't specifically require either biology (which would be primarily bioscience or health professions degrees) or French (which would be essentially just French degrees). So whichever he has a better chance of a better grade would be the best option there (although A-levels in languages are known to be quite intensive and require a lot of work so I wouldn't necessarily consider an "easy" option!).