Please accept my condolences for this tragic loss.Thank you for sharing this painful experience, for the lessons to be gleaned from it.

From the scant information provided, two likely scenarios suggest themselves, purely speculatively. First, death due to exacerbation of a pre-existing or pre-disposed medical condition, such as by a heart attack. Second, insidious hyperthermia and/or dehydration. Each of these can also potentiate the effects of the others.

A bike may make the rider less sensitive to the development and effects of exertion, dehydration and overheating, because of the evaporating, cooling and refreshing effect of passing air. But when you stop, you lose that effect, and your core temperature can spike dangerously. This is seen at triathlons frequently.

Dehydration and hyperthermia can also impair judgment, making self-rescue and remediation difficult. He may have been overcome before he realized he was in real trouble.

Typically, it takes about two weeks to achieve a reasonable degree of acclimation to heat. But acclimation and heat tolerance can be influenced by many intrinsic and extrinsic factors

I think it is far too early to say much about the details of planning, and the actions of his companions, but please do report more of the story as the facts are developed, if you would like to.

My prayers will include those those touched by this sad event.