No, the solution to being accused of metagaming is to stick to your character's established personality and mentality, which Green did.

Having an insane character suddenly start functioning as a sane individual would be no different than a sane character suddenly starting to function as an insane one with no IC trigger, both would be metagaming. For instance, if the Joker decided to turn over a new leaf and become a hero alongside Batman in the middle of a fight because he suddenly felt immense remorse for the atrocities he's committed, it would be metagaming. The same would be true for Batman joining the joker. Neither action is in character, and would therefore be illegal.