Basically the RM can't do anything about him.. Yet.
However, if he suddenly started attacking people for no reason, personal things, or out of RP things, RM could declare him a traitor to Rheinland, now excommunicated, and available to be destroyed by other lawfuls. Such would be RP-correct in response to his unlawful attacks, and RM couldn't be penalized for it. They would be doing a lawful action, the other would be doing unlawful actions. The act of doing anything unlawful classifies one as unlawful and able to be destroyed by a lawful, regardless of the NPC faction he claims membership to.