Currently Being ModeratedJul 8, 2013 12:52 PM (in response to etchaskesher)
There are several possibles -some of which you post would seem to exclude.
However they are:-
1a) The Apple ID on the Device has been removed an you are not sending to the iPhone Number.
1b) The person has two devices and has removed the Apple ID or the iPhone Number formn one of them creating a sort of "Unlinking" of the IDs at the iMessages server.
2) They are not on WiFi. The iPhone in question is then using the Phone/Cell Carrier service which will be SMS
I get this when I send my wife messages when I am at work as I do not join the WiFi network there.
3) The iPhone may list the Apple ID but it has some how become deselected.
I have had this myself. I had not changed anything but the iPhone had the Apple ID Unticked.
Nothing I did could get the Apple ID ticked.
For your Contact to do
For 1) and 3) on the iPhone go to Settings > Messages and remove the Apple ID.
Now put the Phone in Airport Mode or turn the Messages App Off. (break with the iMessages server).
After a few minutes wait turn the Phone/Messages back on again.
Check the settings in the Messages app and check the iPhone Number is actually ticked (it will remain greyed out).
Then re-add the Apple ID.
Soon after this or when you restart Messages on the Mac you should get a pop up to Confirm the Apple ID and iPhone Number are being sued by the iPhone.
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.4)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously