Well, I have no experience with carbon road bike frames, but I got a trek Y frame ATB years ago. I think it must have been a '97 in '98. Whatever was the last year they GLUED on the rear shock. I wanted that bike so bad. Anyway. In about two years it sheared. I never abused it more than normal ATB abuse. I don't know how to describe the shear but it was sorta where the downtube would have been above and infront of the crank. It was very visable and you could feel it on the bike. As far as I know I never hit it on anything sharp there. At least I got the bike cheap. The mechanic said he figured it was my size (not huge, 200ish 6' 2") combined with his insistance that I must have kept it in sunlight which he said would somehow weaken it or something. I don't know if I did. I kept it inside, though.
Anyway. It did fail, but not catastrophically. I just got on it and went off a curb and it rode all wrong. LBS said I was lucky it didn't break during a drop or something. It scared me away from carbon, though I don't think I'm "scared" of it anymore. I think perhaps the earlier carbon frames weren't as well built as they are now. Though I don't know if that frame shape was actually better than a diamond in carbon. I guess I would just be cautious, but at the same time I do know people who have beat up carbon framed bikes and not broke them.
I guess I can sympathize with the guy, I don't think he's unreasonable. Really is a pickle. There's no way you can know if it will or not, as was already implied. Is he waiting on you to say it will last foury more years, then he'll buy it? Otherwise no sale? Sorry to not really be helpful. I suppose I'm non the "carbon cautious" side of the wall.
edit: wow, I was looking for a picture on google similar to how my trek broke and found this (most is crash damage, which is irrelevant, but some is fatigue):
http://www.bustedcarbon.com/2008_11_01_archive.html
I know we could probably find even more pictures of broken metal frames, but maybe this is slightly relevant? Maybe carbon is just too fine tuned for the rougher, heavier riders?