How to remove Activation Lock

See this link, the original owner needs to use apples iForgot service to get in to the Apple ID and remove activation lock or they can ask Apple to do it with proof of original purchase

How to remove Activation Lock - Apple Support

If they forgot the Apple ID (iCloud) password, have them enter the Apple (iCloud) ID, and click Forgot Password. See if they 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 they can recover their password and sign in, have them do the steps in the article below.

Selling/Giving Away Device.  *