Problem

See Turn off Activation Lock https://support.apple.com/en-us/HT201441 See this heading If you erased your own device. It says: "If you don't sign out of iCloud before you put your device in recovery mode and restore, it might remain in Activation Lock. That means you need to enter the same Apple ID and password that … was used when … [the device was last set up]."

Since the guy doesn't remember the needed credentials, you can try to recover them: 

See: If you forgot your Apple ID  https://support.apple.com/en-us/HT201354 and

 If you forgot your Apple ID password https://support.apple.com/en-us/HT201487

You also can use Having trouble signing in/iForgot https://iforgot.apple.com/password/verify/appleid  

If you have iOS 11 or later and your account is protected with two-factor authentication, you can turn off Activation Lock with your device Passcode. Tap Unlock with Passcode, then tap Use Device Passcode."

If you can't recover the needed credentials, the phone will be useless to you.

If necessary, if he is the original owner, have him contact Apple ID Support. Apple Support will need to see the original purchase receipt with the serial number/paperwork.

Apple ID - Official Apple Support.    

Contact Apple for help with Apple ID account security - Apple Support.  (Link doesn’t always load)   

Get Support      Work through the screens to get a chat option. 

Apple Support schedule phone call

https://twitter.com/AppleSupport