I am not sure whether these two examples using singular they to refer to a specific, singular referent are acceptable in educated speech:
I had a friend in Paris, and they had to visit the doctor for a month.
Here, they refers to a friend in Paris, so clearly a person well-known to the speaker and so of determinate gender.
A teacher asked me to give their book to John.
Here, their refers to a teacher of the speaker’s acquaintance, presumably also therefore of known gender.
Specifically, I’d like to know whether there’s any difference in acceptability between how speakers of American English view such usage compared with how speakers of British English view this.
Does the Atlantic change how this comes across, or doesn’t it?
(editorial additions for broader linguistic and sociolinguistic focus)
Does one side of the Atlantic Ocean find it pretty normal for educated speakers to use grammar like this, but the other side of the Atlantic Ocean finds it abnormal in educated speech?
Or does — if you would — the “Atlanticity” of locale have less importance on its acceptability than the register being used has, such as the varying registers of casual language versus formal language, or of the spoken language versus the written one, or of spontaneous dialogue versus curated text meant for an educated readership?
Might the acceptability of usage vary more by other factors than locale or register, such as by gender or age or education or class?
Has this usage’s acceptability or unacceptability remained relatively constant over time, or have we seen shifts in that? If so, are these shifts ones of centuries or of generations, or are they quicker than that, perhaps of decades or even of years?