Is that POSSIBLE? Can someone with a true mental illness like this have that much control over changing their mental state?
I see this with my schizophrenic mother sometimes. I think that it's not so much control over their mental state per se, but rather the cognitive ability to put their own desires ahead of their symptoms.
My mom believes that people she knows are replaced by impostors. She is able to detect them due to subtle differences in appearance that only she can see (because they are caused by visual hallucinations)- moles, an inch or two difference in height, size of feet or ears, etc.
She refuses to interact with what she calls "other versions" of people that she knows. Has actually cut off contact with me for months at a time, even as long as a year once. The night my DS5 was born she hadn't spoken to me in months. I called to tell her about the birth of her grandson she said, "Congratulations young lady, whoever you are." It was devastating.
BUT
If she needs something- lawn mowed, a ride home from surgery, whatever, she can put aside her principles and accept help from an "impostor" in order to get her needs met.