The discussion over at CrackBerry got a bit out of hand, but the bottom line is they'll have to do a factory reset due to the fact there is no Device PIN to fall back on. In BB10, when setting up Picture Password, it forces you to setup a Device PIN as well. I seems the PRIV does not force this, so if you only setup a Picture Password and forget it, you are out of luck.
Maybe an oversight by BlackBerry on this setup, or maybe intentional... but a lesson to be learned for sure.
The user guide linked here recommends this as well:
http://help.blackberry.com/en/priv/5.1/user-guide-pdf/PRIV-5.1-User-Guide-en.pdf
"Picture: A picture password requires you to move a number (that you choose) to a specific location (that you choose) on a picture (that you also choose). A picture password provides a convenient way to unlock your device and is difficult for someone else to figure out. If you set up a picture password, you must also set a backup screen lock in case you forget your picture password combination."
Good to know.