I would agree with you in most circumstances, but in this specific situation it was not an issue. Plus, you're making arguments or assumptions counter to what I have previously stated. The Captain of the other boat did not send out a random distress radio call and take off, hoping someone would happen to show up. He specifically contacted our boat - which he could see - and asked us to come over and assist. And as it is more or less stated, he did not leave his dive site until he had visual confirmation (and obviously a verbal response/confirmation from our Captain) that we were on our way to his position, which wasn't any great distance. Granted everyone was lucky all around that we just happened to be in a position to respond immediately.
Truly there was some risk no matter what. Obviously he made the judgement call that given our proximity, the greater danger and priority was to the man with the medical emergency. Could it have gone badly for the divers he left behind? Possibly. There was no perfect solution, and while I can't say that it was proper procedure or not, at the time the consensus was that it was the best course of action given the circumstances.
I did misunderstand somewhat. I'm not sure how I would have handled it with that info. I know in the Key's, the coast guard can usually get to us faster than we can get to shore. Since they have pretty good medical attention, I'd probably have contacted them and asked for instructions. But, I don't know.