Restore used iPhone

Unfortunately, there is no way to bypass the activation lock. Please ask your friend to try to reset his password. For more help you can contact with Apple Support.

Please see the article below.

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

See if he can recover the ID/password. An iCloud e-mail address is an Apple ID. Use every email address he can think of.

Apple ID - How to find your Apple  -  

Apple ID - Find  -     

He can also go to iCloud.com or appleid.apple.com and see if his Apple ID prefills on the sign-in screen.   

If he forgot the Apple ID (iCloud) password, enter the Apple (iCloud) ID, and click Forgot Password. See if he can get a Rescue or Notification email option if necessary. Link to contact Apple Support at the end of the 2nd article.

Apple ID - Manage

Apple ID – Forgot Password  *      

If he has 2 Factor Authentication enabled, if he can't recover the password using the steps above, he will need to use Account Recovery. The Recovery Process is an automated process and the length of time it takes can vary a lot. Apple ID Support can only answer your questions. They can't intervene in or speed up the process. The article below has some suggestions to shorten the time the process takes.

Apple ID - Recover your Apple ID with two-factor authentication account recovery.   *

If you have changed your password, have you followed the steps in this article on all devices/computers? 

What to do after you change your Apple ID or password - Apple Support.  *