Okay, let's try an acid test.
Rent another tank from the LDS. Take it, use it, and return it. However, before you return it REMOVE the o-ring.
My thinking...yes, it is conceivably the diver's fault for not "inspecting the rental equipment". However, I can see the diver's point, too. The tank was filled successfully, so it's not a long leap to expect that it should be useable upon delivery. True, the diver should have had a backup. Still doesn't detract from the idea that he expected the equipment to be serviceable upon receipt. For the LDS to absolve itself of any responsibility at all means the owner/operator must believe that o-rings are the responsibility of the customer.
Therefore, if the LDS owner questions the missing o-ring on the returned tank (assuming he inspects the tank upon return and discovers the ring missing), the diver is justified in saying that it's not HIS problem. Since o-rings are the in the renter's domain, he/she can do anything with them they choose.
The "common sense" solution? Diver's should be prepared for just such eventualities. However, it is also common sense that an LDS operator should be willing to bend a bit in order to preserve customer relations. Store credit (or even a 50 percent discount--since there was blame enough to go around) for another fill would have been a satisfactory compromise.
But that's just my opinion. While I don't have scads and oodles of dives to "quantify and justify" my view, I have been a citizen of the planet for quite a while and I've dealt with a LOT of people. That should be enough to allow me to voice an opinion on the nature of fair play without labeling me as either "stupid", "arrogant", or otherwise unworthy of having a say in the matter.
Have a great day!