A used engine is probably going to be the cheapest route. It's also riskiest. If you install it and fire it up and discover it burns oil like mad, they'll be happy to refund the money or exchange it. But you eat the labor and the fluids.
Before he (you) spends long green on an engine and all the nickel-and-dime stuff he'll need like belts and hoses and tuneup parts and water pump and fluids, look at the rest of the truck. Is it worth it? A new engine doesn't give you a new truck. If it's got rust and body rot, it'll still be there. If the axle seal leaks and the rear brakes are contaminated, they'll still need a couple hundred bucks' worth of parts. The worn shocks, the leaky rack-and-pinion, they'll all still be there.