What do you do when you run into your friend on their birthday? You wish them a happy birthday, of course! Or wait—you wish … him or her a happy birthday? When you run into your friend on … his or her birthday? That’s how you’re supposed to say it if you want to avoid using they to refer to one person, but it’s a bit wordy and awkward.
Sourced through Scoop.it from: mentalfloss.com