It is incorrect. It is a construct of the latter-day PC crowd who do not want to “offend” us women by requiring one sex or other when speaking singularly. It does not offend me to assume “he” instead of “they”.
Hey, lookie here! Instead of merely taking y’all’s pedantic word for it, I actually RESEARCHED IT! And it seems you’re on somewhat shaky ground:
http://motivatedgrammar.wordpress.com/2009/09/10/singular-they-and-the-many-reasons-why-its-correct/
My favorite retort: “Singular they”: God said it, I believe it, that settles it. http://itre.cis.upenn.edu/~myl/languagelog/archives/003572.html
Gee, that doesn’t sound all that collectivist leftist to me!