What Causes Unconsciousness Post Hiking?

My friend hiked a mountain today in Maine and later this evening at dinner he suddenly became diaphroretic and on the way out to get some fresh air he passed out briefly. He was able to sit in a chair and walked on his own to a bathroom where he had a bowel movement. He said he felt nauseous but was able to walk on his owns hen we got back to our home. We thought he was dehydrated, even though his wife said he drank a lot of water and gator aid.