Heat stress can hit you so quickly up in the Northern parts that virtually one minute you're OK the next you can pass out. Had a few of my mates at work (electrical industry) go over like that, scary when it happens as you have to get them into the shade & cool them down very quickly. 5 litres of water each a day is the minimum we carry at all times & we even go through more some times.
Poor bugger, only trying to help his mates out!
My condolences to his family & friends
Regards
93x64mm