A lesson to be learned about the irrelevancy between experience and evolution, especially in the case where a Pokémon evolved only through Friendship Evolution.
After Lucario had evolved, he was thought to have been an exceptionally strong Pokémon by reputation of other Lucario. But in truth, this Lucario had no fighting experience. So when the owner had foolishly put his Lucario against the neighbor's Riolu, poor Lucario suffered. Even if Riolu did not have the same friendly relation with his Trainer, he had far more experience in battle.

So for all you children out there who received a new pet Riolu or Lucario for Christmas, remember that you should never make your Pokémon battle just for show. Only trained Pokémon should battle.