I wonder what the motivation of being less prepared than possibly necessary your friend has. Might be ego, lack of common sense or just not having faced an eye opening incident before. Sometimes you have one of those eye openers and you think differently about almost everything you plan for.

I tend to agree with Lori's smart #ss ways. My approach might be to plan a hike in a COLD soaking rain. Take him out and if he starts to put on a piece of ran gear say NO, NO, NO, you wont be needing that today as you didn't need it the other day. Let him hike for a VERY short while keeping close to the trail head and then sit and take a break in an open unsheltered area telling him he has just taken a bad fall and can no longer walk on his own. Sit long enough for his body not to be producing heat from activity and when his teeth start chattering say OK I think we learned the importance of keeping dry and head him back to the trailhead ASAP. Now if he's a wise man he will thank you for possibly having saved him from a future hypothermic event.

We all know a wet body is a perfect "cold soak" of body heat and he will surely learn it too. My next move would be to point him in the direction of some good reading on just how easily you can become hypothermic even in the middle of summer. A good article about cold water immersion will explain to him how his body will react when wet and temps start dropping. He may honestly not realize how his body can start to loose its ability to articulate and how his mind will suffer judgment issues as well.

Good luck, jimmyb