Given that the laws in your hypothetical scenario are different, the answer could be everyone or no-one.

Maybe not sending letters saying “hey lol” to people on the WPP is a crime, in which case everyone aside from that guy should be arrested.

More logical, is that Dave already did time for revealing John’s location, so he can’t be done for that again (unless the laws of Imaginaryville allow that) and sending someone a letter isn’t a crime (unless… again, Imaginaryville has laws about that) so no crime has been committed.

Like wtfmcnuggets said, having jailed the guy who revealed their secret, the WPP are pretty ****ing negligent for leaving the guy in the same place. If anyone’s at fault it’s them.