I think I know that you evolve Riolu into Lucario by max happiness (tell me if I'm right or wrong) but what happens if you use a Revival Herb on Lucario?I'm just curious.
Lucario will remain its current evolution because once a Pokemon has evolved, there is no way for it to "Devolve." All it will do is make alterations to friendship based damge.
This applies to ALL Pokemon who evolve through friendship.
It's happiness drops. That's all. Nothing happens to a Pokemon that needs happiness to evolve when Revival Herb is used.