I think the decision hierarchy goes something like this:
Likely to achieve 8 or 9 in Maths GCSE? If so, more likely to thrive at Maths A level.
Likely to achieve a 9 or high 8 at Maths GCSE? If so, take FM as it enables access to very competitive quantitative Econ courses; but take FM as one of 4 at the outset of Y12 in case struggles and need to drop it.
Take Econ as why wouldn't you if seriously considering Econ at uni; look at syllabus and past exam papers online to check pre-conceptions vs what Econ A level actually involves.
You may be told by teachers, parents and others that Econ and Business too similar but check actual uni requs; as far as I am aware, only LSE specifically says it dislikes that combo and only Cambridge says it regards Business as a 'vocational' subject, only to be taken in addition to three academic subjects (Oxford, by contrast does not have such quarms).
If in doubt about selecting the Econ and Business combo, consider Politics as a good fit with Econ but, again, check the syllabus and past exam papers to understand what Politics A level actually involves.
If the prospect is an A in Business but, say, a C in Politics, then it would have been better to ride through the risk of the Econ and Business combo and forgo application to the likes of LSE or Cambridge. However, if a good all-rounder, then something other than Business will give a little more breadth.
Finally, I know a number of people with Econ and Business A levels who went into competitive school leaver accountancy and other business related apprenticeship schemes, so if your DC has an early interest in business, they should be researching that route now.