iPhone thief phished iCloud password. Can they decrypt iPhone contents?
Did the user have an iCloud backup?
If yes, the data is available. 
https://support.apple.com/kb/ph12521?locale=en_US

If no, the phone is erased as part of the recovery process.  To get the data, you would need to connected to the computer the data was backed up on. 
https://everymac.com/systems/apple/iphone/iphone-troubleshooting-repair-faq/iphone-how-to-unlock-open-forgot-code-passcode-password-login.html