This statement is made in Chapter 3 while Hester is up on the pillory for her three hours of punishment.  It is spoken by someone who is described in the chapter as "a stranger."  The guy who is saying it is really Hester's husband.  We will later learn that he is calling himself Roger Chillingworth.  He says it to some person from the town.

What it means is that he does not think that Hester should be up there taking the punishment herself.  "Iniquity" means sin or wrongdoing.  He thinks that the person who commited adultery with Hester is just as guilty of the sin.  Because of that, he thinks that person should be up getting punished as well.