My cousin has always said recently that someone is monitoring her brain waves with some instrument, and she is very skeptical. Said these people tried to kill her. How to treat it?

This may be a manifestation of schizophrenia-delusion of being killed. It is suggested to go to psychological counseling first, and then go to the hospital psychiatric department after psychological counseling judges schizophrenia. The reason why I didn't go to the hospital first is because the hospital only treats you regardless of judgment. Maybe if you are not mentally ill, you will be treated as a problem, and the result may be worse.

As far as I know, this is a typical symptom of schizophrenia, but it is not necessarily schizophrenia, just in case, I suggest seeing a psychologist or other doctors. I am an undergraduate majoring in psychology, and my answer is not necessarily correct. Please have a look for yourself.