I just started reading this thread.
I'm an attorney with at least some relevant knowledge and experience. Thus, a few observations:
1. The husband is unlikely to be paying by the hour. Rather, a case like this is usually done on a contingency basis.
2. That is a good thing as the case is a pretty sure loser. A court in the US is unlikely to have jurisdiction over the dive shop. Besides, there is probably a liability waiver, which would be interpreted under Mexican law.
3 The resort does not own the shop and is unlikely to have liability based on its status as a landlord.
4. The resort is unlikely to have liability for having directed the diver to the particular shop because the shop is an independent entity. Also, there would need to be evidence the resort knew or should have known of some endemic problem with air fills. Since many other divers have not been poisoned, such evidence is going to be hard to find.
5. Re PADI:
(a) How will anyone be able to establish what the deceased was or wasn't taught? Just because something isn't in the book doesn't mean it wasn't mentioned or taught.
(b) The state of the art in 1987 (when te deceased was trained) probably didn't include individuals using CO testers. So what was lacking in her training? Remember, there is no continuing education requirement.
(c) PADI doesn't own the shop, so it can't ve liable as n owner.
(d) The shop is not a franchise, so no liability as a franchisor.
(e) As far as liability for letting the shop use its name, PADI, would need to know of some endemic problem with air fills. Since many other divers have not been poisoned, such evidence is going to be hard to find.
Now, a lawsuit against the shop in Mexico is another story, but it is unlikely to have any assets with which to pay a judgment.
Finally, very few lawyers will take a contingency case where the client says it is not about the money. I was even taught that when a potential client says its not about the money, to politely decline the case.