user12523: Let's say you see that player A on the board next to yours violates a rule such as touch-move or changes the move after releasing the piece, but their opponent, player B, doesn't notice. Can you say anything to player B, and if so, could B make a claim based on third-party testimony for a violation B didn't even see?

user14079: You should not address player B directly, it is forbidden to talk to him about his game (it is actually forbidden to talk to him at all, but the spirit of the law is to prevent conversations about the game). 
However, you can (and should) tell the arbiter about what you have seen, and the arbiter will decide on the appropriate action.

12523: <p>Let's say you see that player A on the board next to yours violates a rule such as touch-move or changes the move after releasing the piece, but their opponent, player B, doesn't notice. Can you say anything to player B, and if so, could B make a claim based on third-party testimony for a violation B didn't even see?</p>
 14079: <p>You should not address player B directly, it is forbidden to talk to him about his game (it is actually forbidden to talk to him at all, but the spirit of the law is to prevent conversations about the game). </p>

<p>However, you can (and should) tell the arbiter about what you have seen, and the arbiter will decide on the appropriate action.</p>
