1991 GMC C1500 Engine intake manifold gasket replaced cause

Long story as short as possible. I had the intake manifold gasket replaced on my 1991 GMC 1500 with 64,500 original miles. Before bringing it into the shop, the truck ran excellent. At the shop, after the gasket was replaced, I started the truck up and there was a new clicking noise coming from underneath the valve cover. The mechanic said it was normal and it should go away shortly. I said ok and drove it about 1.5 hours on the highway. I had to step on the gas unusually hard to get it up to highway speeds. Anyway, the oil pressure dropped slowly and eventually I had to stop or I would of stalled out. When I checked the oil, it was full. I had the truck towed back to the shop and the mechanic said the engine died. The barings were fried. Oil pump is working properly. He's taking NO responsibility for it. Is there any way he could of screwed with something while replacing the intake manifold gasket that caused this? Does the coolant need to be drained before replacing the gasket? Why was there suddenly a rattle coming from underneatht the valve cover? I don't know what to do really? I bring it in, it sounds great. I drive it, and it dies an hour and a half later. Any suggestions?