find old owners email

I got an iphone4 from a friend, it was working perfectly, i even created my own icloud account on it. Something came up and i had to reset everything on the iPhone. After that i turned it on, entered my apple id and password but it didn't sign in. I didn't understand at first until found out that the iphone was already registered to someone else. I asked my friend to contact the person but he couldn't be reached. Now the iPhone is on activation lock. What do i do and how can i get the email so i can contact him?