Rules & Etiquette
What dummy can and can't do
Dummy still has a few rights and one of them is to ask declarer whether he has any more cards when he shows out.
As far as other infractions are concerned, such as leading from the wrong hand, dummy may prevent them if he/she is in time. If declarer is about to lead from the wrong hand dummy may say so.Note, however, that it is only prevention that is allowed: once declarer has led from the wrong hand or has named a card in dummy it is too late to prevent it and, now, dummy must remain silent. Similarly, if declarer puts down a played card facing the wrong way, if dummy can stop him before he lets go, that's fine. Once declarer has put the card down the wrong way dummy must say nothing until the end of the play.
The defending pair, if they see that declarer has led from the wrong hand, may demand that he retract the card and lead from the correct hand. (The card does not have to be left on the table, as it would if one of the defending pair led out of turn). However, they may decide that the incorrect lead is to their advantage, in which case they should continue play and allow the lead to stand.