My daughter has forgotten her login password. I know in the past you could reset the password by booting from the OS X installation disc and using a Reset Password command but now that there are no such installation discs I don’t know how that works under Mountain Lion.
Fortunately this feature is still available but not in the obvious way it once was. Do this:
Restart the Mac and hold down Command-R to boot into Mountain Lion’s Recovery HD partition. From the menu choose Utilities > Terminal, and in Terminal type
resetpassword and press Return.
A Reset Password window appears that lists all the bootable volumes attached to your Mac (see image above). Select the volume that contains the account you want to reset. (If the Mac has just the startup drive, that’s the only volume that will appear.) Locate the Select the User Account pop-up menu in the window and choose the user name that needs its password reset. Enter and verify a new passwords in the appropriate fields and, if you like, enter a password hint. Click Save and the new password is applied to the account.
When you log into this account you’ll be told that the system was unable to unlock your login keychain—that’s because it’s locked using the old password. Your choices are Continue Log In, Create New Keychain, and Update Keychain Password. As you don’t have the old password the Update Keychain Password command does you no good. (As it will prompt you for that old password in order to use the original login keychain.) In regard to a login keychain you’ll want to start from scratch so choose Create New Keychain. This will create a new login keychain where her the passwords she adds in the future will be stored.