Follow The Steps -:
1) At First Open web.whatsapp.com in your pc.
2) Then go to his/her Whatsapp messenger app > options > WhatsApp web.
3) Now scan the QR code which is being displayed in your pc screen and you are ready to go. You can now see all WhatsApp chats of your friend.
4) Do not forget to check ‘Keep me logged in‘ option for future usages.