The previous owner of iphone 4s forgot his account and password, is there anyway to unlock the phone?

They need their Apple ID and password to unlock the phone. There isn't another way.

Turn off Activation Lock - Apple Support.      

See if they can recover the ID/password.

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

Apple ID - Find              

Enter the Apple (iCloud) ID and click Forgot Password.

Apple ID - Manage            

Apple ID – Forgot Password            

If necessary, if they are the original owner, have them contact Apple. They can remove the Activation Lock if provided the required paperwork. Either take the device, the original purchase receipt with the serial number, and a photo ID to the nearest Apple Store (if it has reopened)

Genius Bar Reservation and Apple Support Options - Apple.    

or contact Apple ID Support. 

 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       People have posted it may take an hour or more to get a response.

Did they try the links I posted above to see if they could recover the ID and password? If they still have the original purchase receipt, did they contact Apple?