90 year old woman losing 6 lbs in a week and has brown vaginal discharge for past 4 days... Refuses to go to hospital - serious?

Probably. She has endometrial cancer(the lining of the uterus) until proven otherwise. However her wishes to decline evaluation should be respected. If she is not mentally competent to make such a decision, her power of attorney or next of kin should make that decision for her, based on what she would want if she were thinking clearly.