Nick Diaz is a very talented boxer in MMA, and could do quite well as a boxing athlete. However if he were to fight Roy Jones Junior in the ring in boxing, I would have to favor Jones, who is one of the greatest ever. Sure he has lost many steps over the years, but this was his life, still is, and he is the GOAT in boxing in some peoples' opinion.
Now, Diaz has cardio and great boxing skill and youth vis a vis Jones, but I still feel the skill level of Jones would be too much for Diaz to deal with. Jones would take it.