Hopefully he can find the log books...if not that's a big hit on the value. I'm sure he can, want to stay positive!
As long as you've got the go ahead to open it up and look around:
1) Check for any obvious damage/wrinkles
2) look at any unusual/non factory rivets in the belly or around the attach points/gear boxes.
3) from the pics, the airplane has all the markings of a fully float kitted late model; just pull some inspection panels on the wings and look for corrosion in the in the bottom skins where they meet ribs. It should be all corrosion proofed (green)...but look for differing shades of green. Might indicated later repairs. Look at the cables...should be stainless.
4) pull some inspection covers in the belly and in the rear near the tail cone. That's a good first pass for corrosion; bring a mirror and a light.
6) look really closely at the cowl; make sure it's nice and straight and doesn't have any battle scars. Not the end of the world if it did, it's just expensive to make right. Lot of float planes have some dents and dings due to docking/fueling issues.
7) Look at the tach, it would be interesting to see but that's certainly not gospel

Look under and around the battery. I've seen a skywagon absolutely ruined by a battery that froze and leaked and was let sit.
9) Since it's a late model...it may or may not have bladders. Most had wet wings after 79, some float planes were optioned or modified with bladders.
Ultimately (to me)....you are just looking for deal breakers. Anything more than minor surface corrosion and minor dents and dings and i'd walk away if you are looking for a nice plane to keep a long time. Some will disagree....and I totally understand. we all want different things out of our planes.
Good luck, wish I was closer to help lend a second set of eyes.
59SC