I agree with both of you. I don't think anything should be expected. I also think that some kind of compensation for a bad trip would take the sting out of spending good money for a bad experience. But most importantly, I think that the dive shop won't do anything of a compensatory nature.
Cheers -
Is that your assessment of the customer service level of this particular dive shop, the dive shop as an industry, or the destination? If it was me, I wouldn't demand or expect compensation but I wouldn't be using this dive op again, either. If they believe it was a rare exception and want me to give them another chance, it will be on their dime not mine. They can take the short term gain and lose a customer, or take the short term loss in order to retain a customer. Their choice.