In this case, the equities of the situation really seem to fall in favor of the diver. He had a major, life-threatening problem that everyone agrees is not common or generally detectable, has it documented on video, and went to the shop with an undoubtedly reasonable request (refund) before going public.
I can't fault the shop for not willing to take full responsibility - after all, who knows what happened in reality? Maybe Robert Goulet snuck onto the boat and untorqued the second stage before the dive. But in light of the reasonableness of cjust's request (supported by the equivalency of SSI's proposed solution), IMO the shop should just say, "We really don't know what happened and we can't take responsibility, but we're sorry you were unsatsified with our service and that we're losing you as a customer, and would be happy to refund your service cost." That should be the end of the story. The shop should really count their lucky stars that the OP chose to vent about the incident on an informal forum rather than file a costly negligence suit (meritorious or not, and of which 30 minutes of a lawyer's time would exceed the total cost of the reg service).
Other than that, I join the others in sharing my relief that cjust emerged from this accident unscathed, and kudos to him for keeping a cool head during a very frightening situation, and to the other divers there for showing us how the buddy system is supposed to work!