3/4 tank of gas burned 2004 F250 in Iceland

[Ekortsrewop]

Not me but a fruitcake friend of my wife here in Iceland had his truck filled somehow with gas? He drove it down to 1/4 and then filled it with diesel. He picked us up at the airport (this happen 2 months prior and he spent almost $8000 to keep it running) and the truck seemed to run ok until under a load where he had to let off the accelerator and feather the pedal. I spoke with the mechanic but his english and my Icelandic are way off. I asked if he did a compression test and he said "yes" but we couldn't understand each other. Most of the work performed was on the fuel system and he says he needs something for the #6 cylinder and its not any kind of seal or gasket? I looked online at a few sites and see nothing other than seals. I asked him if it was an injector and he thinks so???? He said the part cost the equivalent of $850 here but from past experience I could buy it in the US for 1/3 the cost or less.... There was diesel all over the motor and the airbox was snapped back together incorrectly so I took the truck home one night to clean it a little and fix some little things these couple different mechanics put back together. When I started the truck cold it was the worse sounding engine I have ever heard including many demolishion derby cars I've ruined. Without any diesel knowledge this sounded like 3 times worse than any main bearing in a gas motor I've ever heard....Last night another backyard mechanic was going to install a fuel pump in the fuel tank (I think or wherever, so much is lost in translation) Do you think this could be a fuel system problem? The motor doesn't sound bad when started warm or running. Because of lack of money I suggested a block heater because although its not so cold here in Reykjavik he lives about 5 miles away and its very cold there overnight.