I would want to know more specifically about the nature of the parent’s illness, the status of his/her medications, and the prognosis for the future. I would certainly take her condition into consideration when talking to her. However, I would also refrain from dismissing her credibility and competence, since ‘mental illness’ is of course a vague category, with many levels of competence etc. Many people with, e.g., bipolar disorder are competent as long as they get their meds. It would be be essential to get commentary from others around her, and her physician.