The Montana State Hospital at Warm Springs was an inpatient psychiatric hospital until deinstitutionalization took place in the late 70's. Now it still operates, but typically as a short term institution. The area where your relative is buried would most likely be one of the cemetaries on the campus as many people who were admitted to the hospital lived there until they passed away. I hope this information is helpful to you. If you Google search for history on the hospital there is a website that you can apply to in order to get records of patients from this time period and their admit and discharge information.