Is it running on straight #2 diesel? If it is running a high concentration of bio, I would not be surprised if it is gelling but on straight #2 it seems unlikely. I know it sounds obvious but it does have fuel right? When he is cranking, is there any black smoke coming out of the tailpipe? If there isn't, he isn't getting any fuel into the engine. It would probably be worth checking the fuel filter to see whether it is plugged and also to make sure that there is fuel there. It is possible to get air in the fuel lines which makes the truck not want to start. It may also be possible that your lift pump is dead and not supplying the injection pump with fuel but I am not sure about that. Is there anyone else who knows more about 24V's that can chime in on this?