I wonder how many had no maths at all?
Most common A levels in addition to geography in the 2013-2017 data:
Mathematics Taken by 45% of applicants (Total 459, Shortlisted 392, Offered 151)
Biology 29% (300, 229, 92)
Economics / Economics and Business 26% (266, 194, 64)
English Literature / Language 24% (244, 171, 65)
History 22% (222, 152, 60)
Chemistry 15% (151, 127, 52)
Physics 9% (92, 76, 32)
French 8% (80, 70, 26)
Religious Studies 6% (63, 40, 16)
Psychology 6% (61, 33, 10)
Government & Politics 5% (51, 33, 11)
Art (Fine / & Design) 5% (49, 29, 6)
Spanish 5% (48, 37, 11)
German 2% (23, 20, 10)
Geology 2% (19, 12, 6)
Business Studies (not Econ & Bus) 2% (17, 4, 1)
Classics Latin 1% (10, 9, 1)
Sociology 1% (10, 5, 2)
As @titchy says, while it is a statistical fact that people who did FM A level were more likely to receive an offer, we cannot conclude that it is doing FM in and of itself that caused them to receive an offer and hence that it is an advantage.
Otherwise, we would also conclude that doing German A level confers an advantage, but doing French A level does not and doing Spanish A level is positively disadvantageous. Yet even just thinking logically, there is no reason why the Oxford geography department should especially be looking for people who can speak German.
There are lots of other things that might account for the higher proportion of people with FM who were accepted, including that they were more likely to have taken 4+ A levels (and AS levels at the time).
You also have to look at combinations of subjects. The numbers are far, far, far too small to draw statistically significant conclusions, but a quick model suggests that there is almost no additive value in doing FM if you are doing physics (i.e. the proportions with [Geo, Phys, M] and [Geo, Phys, M, FM] who get offers are almost identical), but there is if you aren't doing physics. But again, we are talking about tiny numbers of people with lots of other things that may have influenced their chance of getting an offer.
He needs to think what he really want to study. He can't expect to busk his way into studying geography (or engineering, history, business, etc., etc.) at Oxford on a promise that he is really clever. Realistically, if he wants to study geography at Oxford, he can do almost any A level subject and his chances of receiving an offer will still depend more on his grades, his interview, his personal statement, etc. There is no magic bullet. Except geography A level. He really should do that.
(the one outlier in the data really is an outlier. The other five applicants without A level geography were: 1) actually does have geography A level listed as a prior qualification; 2) an international student who possibly could not take geography; 3) and 4) the same person who retook and reapplied but was shortlisted on first application but not on second, had geology A level and geography AS level, 5) had geography AS level, not shortlisted)