"them", "they" and "their" for a single person

are ungrammatical and confusing

because those words are essentially plural.

The transgender world requires using

some new words for the new situation -

something singular just by fusing

"he" and "she" to breathy "shhe",

"him" and "her" to "hem" and "hir"

and "his" and "hers" to "hirs".

A little practice will make it clear(s) !