Presumably this just comes back to whether or not it is in the public interest to tell them. The laws exist (or should exist) because it is in the interest of the general public to follow them, so if someone admits to doing something that seriously contravenes that, perhaps its the duty of everyone to help stop it. Arguably the police are there to "police", but its everyone's responsibility really. The police just help enforce the law, they are useless if nobody respects it.