I brought a 2nd hand phone the original owner forgets his apple id how to solve

If possible, ask for a refund.

See if they can recover the ID/password. Use every email address they can think of.

Apple ID - How to find your Apple  -  An iCloud e-mail address is an Apple ID.

Apple ID - Find  -       

If they forgot the Apple ID (iCloud) password, 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 necessary, if they are the original owner, have them contact Apple. Apple can remove the ID  if provided the required paperwork to include the original purchase receipt with the serial number on it. 

Request additional support for unlocking your device.    -

You used to be able to take the device, the original purchase receipt with the serial number, and a photo ID to the nearest Apple Store (if it has reopened) and have them remove the ID. This may have changed, so have them call the store before going to make sure they can help.

Genius Bar Reservation and Apple Support Options - Apple.