I have a really hard time blaming the 15-year old. In your scenario, he didn't check his gauge, which he was probably taught, and yeah- he screwed up. But he should have never been in the position where that screw up would have cost him his life. That was his father's fault.
But what if he didn't screw up: what if his reg free flowed? He may not have caused that. What if an o-ring burst or something else happened? There are lots of what-ifs.
At 15, with the internet, if he was truly interested, he probably could have found out a lot more. Or, he could have trusted his father (as many teens whose fathers are taking them to do cool stuff will do) and NOT EVEN KNOWN that cave diving requires more training than what his father had. Until I decided to learn to scuba, I had no idea there was such thing as a "scuba certification" much less levels of scuba certification. Sure, there are classes where you learn to scuba; but there are also classes where you learn to quilt too. It is totally unnecessary to take those classes. I'm a self-taught quilter with no problems, but I've seen on message boards people who are "scared" to try something without a class. I'm smart enough to know I would have problems if I was a self taught scuba diver, but if you have a trusted authority figure telling you that you are fine, you really don't know what you don't know.
Without a doubt- the entire incident (I honestly don't know if I can even call it an accident) was the father's fault.