Jumping down someone's throat about it is stupid - unless they're being malicious, then jump away and tear whomever a new one. They aren't technically wrong, though. "They" is an ideal word that's been correct in both the singular and plural sense for centuries. Given more recent social developments, it's an easy way to be inclusive and not "risk" being wrong.
(I'm assuming you used literally "he/she" to refer to someone of unknown gender)