Andy:
First my condolences... I am truly sorry for your loss.
There have been some great comments, and observations.
Regarding the PLB with GPS. Both are great devices, but once the affects of the heat had hit him, it is my opinion that it was already too late. Heat stroke and heat exhaustion can and will make a person of great intelligence not think clearly at all.
Being an avid Mountain Biker and one who loves to push himself a bit, it is my feeling that the thing that could have possibly saved your friend from his tragic demise would have been good group leadership/maintenance.
Often we as adults don't like to do the "silly" things that our teachers and camp leaders did when we were younger. As adults we don't like to stop and do a head count, we don't like to stop and ask if everyone has had a drink and look into the eyes of the people in the group and see that they are there. Why should we do such a thing? After all we are adults, right? The answer to that is pride... Someone can be suffering or having a hard time and "not want to slow down the group" or "not want to be a burden on the group" so we might not mention that we have a killer headache, or have stopped perspiring and have a funny feeling in our spine/extremities,, ect..
Heat exhaustion is a very very sneaky killer... by the time your brain goes, "I think I am in trouble!" it is almost always too late.
Thank you for sharing your story and I hope that our words can help you find closure to what has happened to your friend.
Perhaps we can bring some good from this and be reminded to monitor the groups we are in as we venture out into the wilderness. Being on this site and part of the forums tells me that we here are either experienced or seek to become experienced and at the very least care more about our own survival than the average person.
So, we need to take the lead when venturing out into the wilderness with groups. If you are not the leader take a supportive roll (aka second leader) and keep an eye on the people that are in the groups you are in. Inventory the members and keep an eye on how they are doing. If you notice someone who is having a hard time but seems to be trying to "suck it up" it is our job to accommodate them.
It may not seem the macho thing to do but when I see someone lagging behind or having a hard time I hang back and invite them to hang back with me, allowing it to look like I am having a hard time, or just not doing my best, so that they look like they are helping me, the whole time I can keep an eye on them and force the group to slow up, or accommodate, for me the whole time I can help out the person who is struggling, and they save face.