What if the previous owner forgot their credentials and your are just stuck with a locked iphone?

A while back a friend of mine gave me his old iphone 5s, because he switched over to android. It worked fine then, but it kept popping up please sign in to verify your icloud/apple id. I asked him and he said he didnt remember and didnt write his email and password down. So i continued to cancel out the sign in to verify. After awhile it got really annoying, so i erased the phone completely not realizing it was going to have an activation lock, because his icloud was still connected to the phone. He doesnt know his credentials so now i cant even use the phone at all. please help just wanna get it unlocked.