Its very simple mate, answers these questions and we can go forward. 1) does the person have a smartphone ie ideally android or ios for this exercise blackberry will not work. 2) Do they reside with you? If so do you access the internet at home via wifi (this is the easiest way to do what u need to do) 3) you will need an android phone which is rooted (google rooting if you don't know what that is) 4) download whatsapp sniffer and install it on your phone. It will then sniff packet data coming from the phone which is on the same wifi network as you. The loophole in this, is that you are not contravening "legally speaking" any communications act, as your ROUTER is acting like the cellular providers tower, which if I am not mistaken is not going to get you prosecuted. Obviously cyber laws change everyday, and you would need to post up a disclaimer stating what your actions are, to avoid any comebacks. Good luck with your exercise and here comes my disclaimer. I am not held responsible for any action or actions you perform on any third parties or any losses incurred in doing so. I strongly advise against it and suggest you only do it as a training exercise with someone who is aware of your actions. Best of luck.