What about the person on the other end of the conversation? Does he not share some responsibility for this? If the conversation turned to sex with 5-year-olds, I'm thinking his hands aren't too clean either.
"...The other person involved in the conversation then notified authorities..."
I'd let the informant get a pass, unless this is slander or something. We shouldn't bite the informant on this and discourage other informants in the future.
I remember reading lately of a group of people who get online in forums, and try to get someone to do something like this. They are working with the police.
I do know that some police have officers who do this as their job. Trying to get rid of the filth, one at a time.