Unless I’m misunderstanding your question, you can use:
https://www.inaturalist.org/observations?order_by=random&place_id=any&subview=grid&user_id=[the user ID you’re interested in]
This will bring up all of that observers observations.
Then type the name of the taxon into the Species bar to bring up that person’s observations of it.
https://forum.inaturalist.org/t/how-to-use-inaturalists-search-urls-wiki-part-2-of-2/18792