wedivebc:I don't think a dive operator should make a judgement nor are many qualified to judge a diver's ability. I like Diver Choice Charters approach to this issue and when they book a shop they offer a comp spot for a DM. The shop then takes responsability for the safety of the divers in the water and the boat assumes the role of a taxi driver. Admiralty law assumes the boat operator is only responible for divers while they are on the boat, and possibly on the dock.
I'm not sure if I can agree with that approach, at least for me. I think that if you see a guy that seems like he's getting in over his head that you, as a reasonably prudent operator have a responsibility to protect that diver. I do believe that tact and courtesy are extremely important in situations like this, but first and foremost come the responsibility for the diver's safety.