IMHO, if the jur was eating well in Kahuna's tank, the stress didn't affect it, and the alleged stress therefore can be neglected and the situation.
IMHO a lot depends on your relationship and what was said during the whole transaction. Since we don't know the details of it, we can only approach it formally... and formally I must agree with the vast majority of the guys - once the fish is out of your hands, it is not your responsibility.
Such terms are agreed on before the money changes hands.
There 5 hours passed between 11 am and 4 pm. 5-hour long acclimation? That raises flags. Weird. Who knows what happened - too much and too long a stress or if the addition of new water was that slow, it could have run out of oxygen, or maybe it died in his tank - all this guessing is nonsensical. And, again on paper, none of your business anyway.