Just use the correct one, increasingly pointedly, and hope they get the clue?
As long as it's well-intentioned and in what sounds like a really niche situation, also kinda whatever? In general, in English, they/them can be used for a bunch of pretty decent reasons, like just not being sure which is appropriate. If whoever feels bad about it, that's one thing, and people need to be made aware. Does it need to be assumed they feel bad about it? You could probably ask whatever they'd prefer in this or that hypothetical, on the off chance they've ever thought about it.