LOL!! He was pretty blunt, but he is right. I was thinking the same thing, but just did not have the heart to tell you! The Diesel in the fuel sounded really strange.
First things first. I would take it to a small engine shop and see if the engine is good or not. As Popeye said, they can do a compression check and tell you what you have and then go from there! If the idiot did put diesel in the fuel, the engine may still be good! It may be that the plug is fouled and diesel fuel would certainly smoke.