Microsoft® Manual of Style, Fourth Edition

Agreement

It is easy to remember and apply the rule that a verb must agree with its subject in person and number. It is sometimes more difficult to apply the rule that a pronoun must agree with its antecedent, the person or thing the pronoun refers to.

Pronoun-antecedent agreement is particularly difficult with singular personal pronouns. Writers and editors do not want to give offense by using a singular pronoun of a particular gender as a general reference, so they use the gender-neutral but plural they to refer to a singular antecedent.

International considerations

International considerations

Although using the plural they for a singular antecedent is gaining acceptance, it remains a problem for localizers and for non-native English speakers. ...