....In Matt's situation, if his buddy had been in trouble, I think we would be having a different discussion since his actions would have likely engaged a lost diver search sooner rather than later, and perhaps the diver would have been found before OOA (such as in a case that the diver was stuck). So, I don't see anything wrong with his actions in the water, even though many of us would have acted differently (back to my point that I am not aware of any standard protocol for this).
The only issue I have with Matt's actions, is back on the boat. Once the primary fear of the buddy being in trouble was put to rest, it is time to swallow some pride and apologize if your improvised lost diver protocol was not appropriate.