Goes black? Well, there could be many reasons, but let's try to narrow it down first. When she tries to log off and the screen is black, laptop or desktop.. are the lights (on the laptop or tower) still blinking as if it is still trying to close some apps or programs down? or completely not blinking at all?
Other than that, do you have a registry cleaner and anti-virus/spyware tools? Have you tried running them?