Not everyone is a he or a she. I'm not, for example.
Curious, I looked for guidelines and the best I could come up with was this, which seems helpful if you know the person in question, but not so much if you're referring to someone completely unknown: https://en.wikipedia.org/wiki/Third-person_pronoun#Transgend...
I welcome advice.
Singular "they" has a long history of being used as a generic pronoun, before a bunch of white men decided in the 18th century to try and prescribe "he." Singular "they" is making a comeback, though, and in this case it would be appropriate.