Tell us which version of OS X your Mom is using on her Mac. The procedure for assigning a new password varies. Up to OS X 10.6 the password is changed with the install disk, from OS X 10.7 to the present, it's done from the recovery partition using Terminal.
If you mean the login password this will help. if you mean FileVault bad, bad news. There is no Mavericks disc. The option was to burn to a USB thumb drive prior to rinning the bInstaller. After Installl the Instaler deletes from the Applications Folder.
OS X Mavericks: Reset a login password
FileVault is much more of a challenge:-
OS X: About FileVault 2
What is FileVault and how do I know if it's turned on?