Mont. Code Ann. § 53-21-190

The fact that a person has received evaluation and treatment, whether voluntarily or involuntarily, at a mental health facility may not be admitted into evidence in a subsequent proceeding for involuntary commitment or for the appointment of a guardian or conservator unless it is necessary to a determination of the present condition of the respondent or the prognosis for treatment in the present case and the judge determines that the need for the evidence outweighs the prejudicial effect of its admission.