In my opinion, it's overpriced about $1K! If there's that much rust showing through the paint, the battle will never be won against the rust monster. Most of those cab panels are multi-layer assemblies, and it will be impossible to do more than temporarily hide the rot. Offer him 500 bucks for it, and if he refuses, count yourself lucky! I bought an 85 C-10 in much better shape for $100.00 recently, with a running-shape 305 engine and a rebuildable turbo 350 transmission. My nephew has been back and forth from Nashville to El Paso, Texas in it twice so far.
Jerry