I'm not sure you and your student are logically correct in your desire to move to aa, bb, cc. After 52 notes that leaves you moving to what, aaa, bbb, ccc? This is likely to get cumbersome after you get up into the hundreds of notes.
aa, ab, ac... is using the same logic as decimal numbers 11, 12, 13..., but in base 26, sort of.
Thanks for looking at the post, but I disagree with your assessment for several reasons:
First, Microsoft Word is the elephant in the word processing room and it is hard to ignore its formatting. Thus the request from the author. If Word didn't provide that system for formatting, I doubt the author would have asked for it.
Next, InDesign should offer a customization for the formats of this type of footnotes. Especially since it differs from Words. Hell, there are SO MANY customizations in ID I'm surprised they skipped this one.
Finally, your comment about the notes getting cumbersome after 52 is spurious. If there were hundreds of notes in a story, I doubt the author would have chosen letter indicators. The footnotes stop after the end of a story or the end of a page. Most likely the notes don't go above 30. So it is not unreasonable to simply want the aa, bb, cc, etc. format.
But even if the notes did continue above 52, the references that ID provides are hardly easier to follow to the note at the bottom of the page. They would be abc, bcd, cde, def, efg, etc. This is harder to refer to. At least the aaa, bbb, ccc, etc. is easier to see.
I think the problem is, as you say, that aa, ab, ac,... is using the logic of the two-decimal numbers but in a base 26. That's not the logical solution. It's the logical problem.
I wasn't disputing the user's right to have an opinion or a preference, just pointing out the reasoning behind the system as it stands (and I don't thnk it's a trivial thing to make aa, ab, ac.. vs aa, bb, cc... a user option). Nor is the question of a lot of notes spurious. Just because most folks wouldn't use letters for a lot of notes doesn't mean that you don't have to code for the occasional idiot.
And just because Word does something doesn't make it right. Word also substitutes fonts without warning and makes faux bold and italics. I bet you had the same conversation with your mother that I had with mine about jumping off a cliff just because everyone else was doing it.
Frankly, if we're looking at alternative methods of letter refernces, and you aren't going to have hundreds of notes, I think I'd prefer a, b, c...z, a', b', c'..., but going beyond a double prime can get hard to distinguish, too.