Actually, it's accepted usage now. This from Oxford English...

Actually, it's accepted usage now. This from Oxford English Dictionary: 

2. third person plural singular Used to refer to a person of unspecified gender.‘ask a friend if they could help’

Usage: The word they (with its counterparts them, their, and themselves) as a singular pronoun to refer to a person of unspecified gender has been used since at least the 16th century. In the late 20th century, as the traditional use of he to refer to a person of either gender came under scrutiny on the grounds of sexism, this use of they became more common. It is now generally accepted in contexts where it follows an indefinite pronoun such as anyone, no one, someone, or a person, as in anyone can join if they are a resident and each to their own. In other contexts, coming after singular nouns, the use of they is now common, though less widely accepted, especially in formal contexts. Sentences such as ask a friend if they could help are still criticized for being ungrammatical. Nevertheless, in view of the growing acceptance of they and its obvious practical advantages, they is used in this dictionary in many cases where he would have been used formerly. In a more recent development, they is now being used to refer to specific individuals (as in Alex is bringing their laptop). Like the gender-neutral honorific Mx, the singular they is preferred by some individuals who identify as neither male nor female. See also he and she.