Gasoline will cut diesel. :chuckle:
Sorry, I couldn't resist. This is really a tough one. After stripping the interior. Acetone will remove from all the metal work. Cleaning the upholstery is an entirely different proposition. Steam clean may be worth a shot but I would think that unless this person is willing to let it air out to some degree on its own they're looking at replacing some of it. One of the problems here is that even though you thoroughly break the interior down, the fumes end up in the head liner, front seats, etc. Diesel and 90 weight gear lube are to of the toughest odors to remove that I've ever come across.