Can someone else see my WhatsApp contacts / chats if I downloaded app using his account? I am a new user of iPhone. I got this phone from a friend of mine. He already had WhatsApp installed. I just logged in using my WhatsApp password.

When I was trying to update WhatsApp, I found that this app was downloaded using his account (it asked for an update). I want to know if he can view my contacts and chats?

If yes, then how can I delete that?
He can only view your info if he's logged into WhatsApp using your ID and password.  It doesn't matter under which account the app was downloaded/updated from the App Store.  

When you're done with the iPhone, and before you give it back to your friend, be sure to log out of WhatsApp.