I’ve been chided by an editor for using ‘they’ to refer to an individual. We all do it in conversation all the time. Example: “If any one of you have an issue with what I said, they need to see me.” Of course, the sentence should be “If any one of you have an issue with what I said, he or she needs to see me.”
Thing are changing. If anyone has an issue with the correct using, I ask they take a look at the article on How ‘They’ Won (https://time.com/5763175/they-as-singular-pronoun/) by Katy Steinmetz and published in Time Magazine.
One of the beauties of using they as a singular pronoun is it being gender neutral. I’m writing a mystery and I don’t want you to know the gender of a bad guy. I have to perform literary gymnastics to work around the issue. Now, they can be either. Long live They.