Provided the opening lead isn't faced, there is no penalty for the wrong defender leading.
He simply replaces the card in his hand. If the wrong defender actually faces an intended
opening lead, the rightful declarer has five options from which he must choose without any help
or advice from his partner.
He can elect to be dummy and make his partner declarer. There is no penalty for
the defence. If the legal declarer exposes his hand he is deemed to have chosen
this option.
He can elect to remain declarer but accept the opening lead, playing next from
his own hand so that the lead effectively runs round to dummy.
He can refuse to accept the lead and require that the correct defender lead
the same suit as the illegally led card. The illegally led card is
returned to its owner's hand and there is no further penalty. If the defender
on lead is void in the illegally led suit then he can lead whatever he pleases.
He can refuse to accept the lead and require that the correct defender lead
a different suit from that of the illegally led card for so long as that
defender remains on lead. The defender may choose any other suit and the
illegally led card is returned to its owner's hand.
There is no further penalty.
He can refuse to accept the lead but allow the correct defender free choice
of opening lead. In this case, the illegally faced card must remain down as a
(Major) Penalty card.