It says he was vomiting, so I’m sure that contributed to the dehydration. I can assure you, when I go walking here in the mornings in the summer, even without the sun up yet, it’s close to that hot. If I don’t drink regularly while I’m walking, I feel nauseous and dizzy. And that is just regular walking. I can see how this could happen to him.
I went to the link and read the full thing. Two other people were offered water. When this guy fell down and said he needed water, it was refused. The guide told him he could go farther than he thought he could, and told him to get up, he could make it. That’s when he noticed the guy was no longer breathing.