Certainly the advice of NOT getting it worked on without telling the owner is correct. Just take it back to your friend, point out and everything will be fine. He'll fix it, have it fixed, tell you he doesn't care, whatever.
I got my Ruger Redhawk .44 Mag in SS and I am very glad I did.
Very good advice about not getting work done without talking about it with the owner.
I too have never seen any part of a stainless steel gun rust from skin oil...not saying it can't happen, but to happen as quickly as you describe seems more than weird to me. I have two stainless handguns and a rifle with a stainless barrel...never had any rust on any of 'em and I touch them all the time. When I clean them all I ever do to the non-moving exterior surfaces is wipe them down with a soft dry rag. The rifle barrel - I don't think I've ever put anything on it at all to "protect" it.
Maybe the owner did something to the barrel. Treated it with something that interacted with the metal? (Stainless is an alloy, lots of possibities exist). Maybe there's a manufacturing defect. Whatever, it doesn't sound possible (IMO) that any rust would be your fault.
Rusting stainless from just touching it? Do you sweat some kind of super acid or alkali? Physiology would say that if you do you would not be alive.:shock:
I realize that Kimber's warranty isn't for life like Springfield, (unless they've changed), but a manufacturing defect - I can't imagine that they would not stand behind something like that even if out of warranty. . Something is wrong with this whole picture. It just doesn't seem possible that you could be responsible for causing rust just by touching the barrel hood. (In fact many of us, including myself, have polished the bluing off the NON STAINLESS barrel hoods of our XDs and I haven't heard of anyone getting rust on them just from touching them).
You sound like a standup guy. You say he's your friend, so he's gonna know you didn't do anything with bad intent. No upside in not being straight up with your friend. Waiting and asking strangers for advice isn't the solution here IMO.
Peace,
D.