Here is ETIQUETTE VIGNETTE NUMBER TWO – Dummy Reminding Declarer Where Play Is—In Hand or On the Board (ref: Law 43A1(c) and Law 42B1),

     a. “Dummy must not participate in the play, nor may he communicate anything about the play to declarer.”  So states Law 43A1(c).  However, “dummy may try to prevent any irregularity” says Law 42B1. 

    b. Here’s the fine line:  how can dummy “not participate” and still “prevent” the irregularity of partner’s leading from the wrong hand?

         1) Dummy “waits”.  Yep, dummy remains silent until it is perceived that declarer is about to play from the wrong hand; for example--about to withdraw a card from hand when play should be from the board.

         2) At that determined point, the scale is “tipped” from dummy’s illegal participation in the play to dummy’s legally preventing an irregularity.

In short: a) play dumb; b) watch and wait; c) prevent at the “correct” moment.