Is the pen lifted from a patient with multiple personality disorder, and are prayer and fasting obligatory for her, knowing that she is able to perform all that the Sharia has commanded except prayer?

The jurists have unanimously agreed that intellect is the basis of religious obligation (taklif), so acts of worship are not obligatory for one who lacks it. If the illness affects the intellect and removes the ability to choose and discern, then the religious obligation is lifted. The assessment of this falls to trustworthy doctors, for not every psychological or mental illness removes responsibility.