native speakers of English automatically rely on singular they (their, them) as the pronoun of choice; for example:. . .I don't care what everyone else is doing; they're not me.If someone calls, tell them I'm out.
. . .strict grammarians -- especially those who control the citadels of formal education and publishing -- dictate that the epicene pronoun they reference plural, not singular, antecedents.
But singular they has a long history of usage. See the bottom of altieri's article and Henry Churchyard's page on singular they/their for more examples.
On a related note: here is a satirical post criticizing the use of plural they. Let's go back to the good old days when the proper plural third-person pronoun was hie.