@ consumer levels, diesel vehicles didn't need to use Diesel Exhaust Fluid until the 2011 year models, regardless of truck brand. The 7.3L-equipped Fords ran from '94 through '03, so these engines are not equipped to carry or use DEF.
Your friend had a valid question, but now you know for yourself and how to answer him.
Regarding "Deleted" - that means different things depending on year model. It could be as simple as what
@CloverLeafFarm said, all the way up to removing things on the engine to removing the factory exhaust. (Simplified answer) Actually more involved, but you get the idea.