It is not generally accepted as good English, however.

"Then shalt thou bring forth that man or that woman, which have committed that wicked thing, unto thy gates, even that man or that woman, and shalt stone them with stones, till they die." [Deut. 17:5] (link)

There are many examples of they used with singular antecedents. Chaucer, Shakespeare, and Austen used it; even the King James translation of the Bible used it.

Ceci n'est pas un seing.