Re: That would still be an offence, most likely.

There's no way that they can prove that he knew the LG was just someone roleplaying, and they'd likely doubt him because he's a nasty pedophile.

If you got a good enough lawyer, the lawyer might be able to convince them that you just thought you were roleplaying,

You'd have to produce evidence that you thought beforehand she was an adult lady roleplaying, such as for example other transcripts to someone else.

Not impossible to pull off, but not possible on just your word against the entrappers. And not something that many people think about beforehand, I think. Would be trivial to.