The trans "they/them/their" is actually a very new innovation
The singular "they" has been in use since Old English, but it's always been either distributive ("everyone" is kind of plural in sense) or indefinite ("somebody"). Maybe five years ago I first saw it used in the new sense as "a mother and their child" -- indefinite, but of known gender -- and now it's come to take on the sense of a definite person (your friend).
TL;DR don't feel bad, it's very new and a totally new use for the word.
This is 100% original "research"/observation so take it with a grain of salt.