How do I unlock someone's iPhone 6 without providing the owner's Apple ID and password?
You can not unlock the owner's iPhone 6 without providing the owner's Apple ID and password, or, if the owner created one, the passcode or password.  Apple does not keep that information, either.

The only thing you can do is to restore the owner's iPhone 6 from a previous backup, or set it up as a "new" iPhone.