Is it correct to use "their" when referring to a single person when the gender is known? [duplicate]

"Correct" is opinion as English is a natural language, and is therefore off topic.

However, "usual" and "accepted" are measurable and therefore valid questions. My experience suggests that the use of the singular they is spreading at an accelerating rate down this list :

Gender unknown (the doctor I will see tomorrow)
Gender hidden (my partner if I choose to hide their gender)
Gender irrelevant (the doctor I saw yesterday)
Unspecified person of known gender (the woman in question)
Specified person of known gender (my girlfriend)

My experience is that as people are criticised more and more for politically incorrect use of he or she but almost never criticised for they except by pedants, people (especially children) are increasingly using they as it's easier than thinking which is correct.

"Correct" is a social judgment, not a linguistic one.

The OED has examples of singular their going back to 1382, but there are still people around who think there is something wrong with it.

If you care about being judged by people who think that, then don't use it. The rest of us will carry on using this useful English word with a long history.

Insofar as anything is right or wrong in grammar, 'their' is wrong here, and 'her' is right. Imagine you were learning English as a foreign language; I'm sure you'd be told that (unless there is gender ambiguity) a female individual requires the possessive adjective "her".

The only reason I can think of for using 'their' in a case like this would be to tease those who are annoyed by such solecisms.

To be short: it depends who you ask. The singular pronoun "them" has been in use since the 14th century, so it would be hard for someone to claim that there's no precedent for using it. Not only is there; it's actually rather popular.

But should you use it? That's where it depends who you ask. The Wikipedia article on the subject has an excellent, well-referenced section describing the advice of various style guides. Suffice to say they disagree.

https://en.m.wikipedia.org/wiki/Singular_they

If the experts don't agree, who are we to answer your question? Certainly singular "they" appears in most any modern dictionary. But some style guides prohibit its use, while other style guides encourage it. As with all decisions to be predicated on opinion in writing, when deciding whether to use it, take the rules of the organization on whose behalf you are writing into account, and choose whichever option is most appropriate for your target audience.