Is it possible to spy on WhatsApp messages sent to an iPhone for free? How?
No you can't. And why would you spy on other phone?
YES YOU CAN HUN
whatsapp messenger uses the phone number of the user so it would be really hard to do that and it will illegal to do so.
One way possible is through wired connection to your computer, you can use known file manager for iOS to locate the conversation file, and then extract the file into xml file.