Dummy cannot draw attention to an irregularity (e.g a revoke or lead out of turn) by a defender in the middle of play.

Dummy may not call attention to any irregularity during play.  (LAWS 43.a1.B)

but

Dummy may draw attention to any irregularity  after play of the hand is concluded. (LAWS 42.B.3)

Any player can draw attention to an insufficient bid, but the next player to bid can choose to accept it. 

A bid that fails to supersede the last preceding is an insufficient bid.  (LAWS 18.D)

    For example, North opens 1♠  and East then bids 1♥ 

Unless probibited by Law, any player may draw attention to an irregularity during the aution period, whether or not it is his turn to call. (LAWS 9.A.1)

Any insufficient bid may be accepted (treated as legal) at the option of the offender's left-handed opponent.  It is accepted if that player calls (LAW 27.A.1)