(source)
“Oh no, someone left their cell phone”
“Dang, i wonder if they’ll miss it?”
“of course they will, it’s their phone.”
“not what i meant. I was wondering if they’ll miss it in time to come back for it before the shop closes”
“I hope so, for their sake”
You already know how to use singular THEY.
When a nonbinary person asks you to use “they” as their pronoun, you can handle it.
Since English is my second language, I have always been baffled by those native English speakers who say that you cannot use the pronoun “they” to refer to one person, as “they” is plural. These are people who constantly refer to me as “you”, a pronouns which also used to be plural (”thou” being the singular). If you can refer to me as “you”, you can also refer to nonbinary people as “they”.