I've had the issue before. You can ask them to verify that iMessage is turned on, you should verify as well. Go to Settings > Messages > iMessage. If y'all are both turned on, then try turning it off for a few seconds, then turn it back on and try it. If that doesn't work, then you can both try to reset your phone (NOT RESTORE) by holding the sleep button and the home button down until the Apple logo reappears. Either of those should do the trick. If not, then I'm not too sure what the issue could be. You can always run by an Apple store and ask one of the people in the red zone (front of the store). If they try to send you to the family room (back of the store) and you are out of warranty, then just find a way to walk away, otherwise they'll try to get you you buy or upgrade. Best of luck!