Maybe the problem here wasn't insufficient training or supervision; perhaps the pilot had learnt too much.
Namely, that there is one set of rules you use in training and for check flights, and another, completely different set of rules you use "in the real world".
I can not imagine that there is any pilot who isn't taught during basic training that you don't fly in IMC as a non-rated pilot in a non-IFR aircraft. If the pilot considered flying under these conditions to be acceptable, then that is something he must have learnt after and outside of his PPL and ATPL training.
So maybe there isn't a lot we should learn from this accident after all ...