If they had provided the dry suit inflator hose needed for safe operation with the suit they probably wouldn’t be here. They do not appear to be principally at fault, but they appear to have helped set the stage.
If they did and the victim failed to bring it then they are in a lot better position.
You make a fair point and I have no idea how the law interprets (or will interpret) the claims and many of the points made here, including yours.
There are many possibilities, from the seller not having such a hose or has one for another dry suit that they are keeping. The seller cannot reasonably be expected to know if the dive shop has appropriate hoses. As it was the instructor who facilitated this sale, it is not unreasonable for the seller to think the shop had it covered.
I still feel pretty strongly that the instructor is still the one who had the authority and expertise to call the dive. The dive center should have had the dry suit inspected to ensure that there was a proper fit. Improperly fitted dry suits are just a disaster.
Also, if the instructor had performed a weight check on the students using even gross methods, the student wouldn't have had 44 lbs.
Given that the student did not have ditchable weight, and with the weight distribution described, the student was probably one of the most, if not the most, foot heavy dry suit scuba diver in history (hardhat commercial divers obviously excluded). Let's assume she did have an inflator hose. From a vertical position, a fully inflated BCD at depth, how much lift would she get from her dry suit if the dry suit is burping continuously? If it had a silicone neck seal, that dry suit would have burped like crazy. A latex wouldn't be as bad, but still would be allowing air to escape. A neoprene neck seal would perform the best, but still likely burp. I've had all three types of neck seals with my previous dry suit.