“They” has been used as a singular pronoun for centuries, since 1400 or so according to the Oxford English Dictionary. When I edit, I take into account the intended audience. If I think my client will get dinged because readers will insist singular they is wrong, I’ll suggest rewording.
From Merriam-Webster’s 11th Collegiate Dictionary:
Writers and speakers have supplied this lack by using the plural pronouns: “and every one to rest themselves betake” — Shakespeare; “I would have everybody marry if they can do it properly” — Jane Austen; “it is too hideous for anyone in their senses to buy” — W. H. Auden.
Also see this blog post discussing singular they filed under “Prescriptivist Poppycock” at Language Log.
