Re:Scrabble rule change allows use of 'OK'
by mattkrause on Thursday May 02, 2019 @08:48PM
(#19814473)
Hmmm, interesting idea.
I looked at some older examples of singular-they and they almost exclusively refer to an unknown or unspecified person ("Everyone loves their mother"). Using it for a specified person seems to be fairly new: Time Magazine had a piece describing how their (in this sense) was the 2015 word of the year.
I suspect it depends on dialect. "their" just sounds more informal to me, "his"/"her" sounds slightly more formal.