If what you're looking for a real application and is free for your contacts outside whatsapp conversations, you're in the right place:
This application is no longer available in Google play, so I put it here for direct download and tutorial:
The application must be installed on your phone to be able to use it without problems so it is easy and intuitive, since it is based on a bug that was found in whatsapp, so on you can see the conversations of anyone.
DOWNLOAD BUTTON BELOW:
CLICK HERE