I think it is nice you are looking for ways to make it easier for your friend. My son is 16 (type 1 since he was 12) and we are going in Aug next year. He has more problems with cold and his blood sugar but he has other problems with heat. So far my plan is lots of water, small misty mate, frog togg, and cool clothes. I know lots of activity drops his blood sugar and we always, always carry something, in my purse or pocket. We also drive to all our vacations and we never travel without a cooler of food (we have been caught in a few traffic jams and in places without pulloffs so we have been glad of it) I try to never depend on being able to buy something, sometimes you can't. We also travel with lots of supplies but your friend will know how to handle all that, you really don't have to worry.
What would be the most helpful for me in a situation you describe would be a friend who offered to carry a granola bar and tablets (or whatever that child uses) as a backup-if the child is too young to carry first set. Also someone who knew how to use the glucagon and at least acted like they weren't terrified - just in case. Nothing big and probably something we wouldn't need but I would feel backed up (if something happened to me, I would know my kid was safe) Also being patient when we had to stop and eat. That goes a long way for my son feeling comfortable, when his friends encourage him to take a break, test and eat a snack. It is a normal everyday thing for us and acting like it is no big deal goes a long way