How do i repair now?

Do not even attempt to start it, drop the fuel tank and get it cleaned as diesel can grow a type of Alge, so all of that needs to be gone, back flush the fuel lines and you should replace the fuel pump, filters and pressure regulator, change the engine oil and filter, put it back together and only add about 5 gallons of Chevron fuel, then prime the fuel system, then hope it fires up. Now siting that long you may also have some electronics that no longer function, like stuck injectors, ignition module and or pickup coils, etc. After you do get it to run then change out all the fluids and rubber hoses. I'll wish you good luck.

Since the vehicle has been parked for several years, with diesel in the entire fuel system and engine, you'll want to see if the engine will even turn over. Remove all the spark plugs and try to turn the engine by hand. Grab the big pulley at the bottom center of the engine and try to turn it once you have all the spark plugs removed. If it will, hopefully, turn by hand then move on to the fuel system and other things, as Gene said. Regardless of what your friend told you, assume it was filled up with the diesel, driven away from the gas station until the diesel got completely through the fuel system, and then it died. Jim