May I ask why? Its known now that a person will never overcome mental illnesses, especially those with more extreme cases of schizophrenia or psychosis. A lot of drugs out there for mental illnesses are very heavy sedater so if a person living with mental illness comes to the point of having no quality of life at all, and would like to end that suffering, why not?
Sorry if I'm butting in, but while there are definitely mental illnesses that are incurable, it's not impossible to have a good quality of life even with severe mental illnesses. I think that there is enough hope to improve your quality of life even with a permanent mental illness that assisted suicide may be an extreme option.