In PRO happiness evolutions don't have the level up part - So a mysterious candy is useless here.
Faint your Pokemon in a wild battle to lose some happiness and then bring it to max happiness again.
If that doesn't work faint the Pokemon again to lose some happiness and then raise its happiness to max again but this time via the Groomer NPC.
You can find one at Goldenrod Underground Path, Cerulean City House 5, Slateport City or Hearthome City.
Good Luck