Corrupted iOS backup - notes.sqlite is encrypted!
Notes Recovery

The Easy Method


Check on iCloud.com - Sign in and Navigate to the Notes App
Check on other emails used - iPhone saves none local (and non iCloud) Notes as emails on the users email server. Navigate to the email login for the email they use and check for a Notes folder. In Gmail its on the left sides bar.


Notes: If The Easy Method works then the client can set up the phone from scratch and sign in with there email. The notes should sync.

The Technical Method


Restore the iPhone and login with the same Apple ID, create a single note.  Backup this phone (non-encrypted).  Navigate to the Notes folder on the new backup and replace the contents in the new backup with the contents of the notes from the old backup. Restore the phone using the new backup.  If it says its corrupted then the Notes are probably indeed corrupted.


The Hacking Method


I suggest heading over to https://security.stackexchange.com for cracking hashes and other things.


Next Time


Make a backup in iTunes normally. Archive it. Make another backup
Make sure all notes are on iCloud or a Email Server 
Use a Mac --> This ones just a preference :)