How do I get Messages on OS X to text to a phone number? I recently rebooted an OS X machine on which I had an ongoing conversation in Messages via text with a phone number on a non-iOS phone. Now, though I am able to continue sending texts from Messages on iOS to exactly the same number, I can no longer send texts to that number from Messages on OS X, instead messages fail to send, the two conversations are no longer synced between OS X and iOS, and I'm told that the "phone number is not registered with iMessage".

How do I get OS X Messages to remember that I can, in fact, text that number?
Check "Text Message forwarding" under phone Settings > Messages > Text. This must be on (and your iOS device and OS X machine must be on the same network) for text messaging from Messages on OS X to work.

(It's possible this was disabled disabled for his Mac after a reboot.)