He lives in Thailand so he knows it often rains in June. He entered the cave only one week before the official start of rainy season. He took a rather large group of kids, not adults who could assess the risk themselves and decide whether or not to take the risk. I would say that adds up to being very irresponsible.
There may be plenty of blame to go around, but I don't think blaming an inexperienced caver is very useful.