There should never be an argument in the first place. The person who disagrees should state what they think, and if the other person still disagrees then the judge should be called by the people playing the game. That is what the judge is there for. A spectator should never say anything unless they catch someone cheating. When I say cheating I mean something like moving a dial to a different maneuver when the other player is not looking, or moving a rock etc. Even then the spectator should not interrupt the game and should tell a judge.
If it's a rules question then a spectator should never get involved. It should be up to the players in the match.