I agree that the kid, in that situation, should have stayed put - however, knowing to mark his trail and recalling a good hi-vis way to do it was clever, so good on him. Kids like adventure, (plenty of times when I was a kid when we'd think we were wandering off into the wilderness with nothing but our bows, a belt stuffed with arrows and a pocket knife and we'd pop up behind our school ) which isn't necessarily a survival trait while out and about - but at least he put some thought into it, and didn't just go running off as many kids would've done!
I also agree that the adults in charge were at fault, in that he should have been better supervised. A whistle is definitely a good thing to give a kid, especially if you're travelling in a group where it's easier for someone to get 'misplaced'. However, it shouldn't be relied upon - sounds can be hellish to follow in a forest, in my experience.
PS: Could hardly have picked a nicer spot to get lost though eh?
All will rise again for a better day; earth, green, with waterfalls where eagles hunt their prey