OK, then, forget my speculation in the previous post. I was trying to present a plausible explanation for something that made no sense whatsoever. We are now left with something that makes no sense whatsoever.
Despite their lack of formal training, everything we have heard says that they have received some sort of training, even if only (at a minimum) reading online discussions. Pictures of them show their gear looking properly set up. They have been apparently not only been diving in caves for a while, they have apparently being doing decompression dives for a while. However they got it, they apparently had at least a basic idea of what they were doing.
The problem is that the details of this dive do not indicate that they had even that basic idea of what they were doing. They went to 233 feet, leaving their decompression gas near the entry point (which is common),and at some point the son ran out of air and they had to share air on exit. I have been reading about this incident on several forums, and I don't see any indication of a gear failure causing that problem. Even if there had been such a failure, following the most basic rules of cave gas management, which they should have known, a catastrophic gear failure on the son's part at the worst possible point in the dive should have gotten them to their decompression tanks. Now, they did almost get there, but what would they have found when they did? According to reports, they did not have enough gas in their decompression tanks to come anywhere close to the amount needed for the required decompression after a dive to 233 feet.
So that means there were two complete failures in the most basic knowledge required to do that dive, knowledge they should have picked up easily in whatever informal training they had received.