Mont. Code Ann. § 37-61-414

Whenever a law partnership or any member thereof accepts employment to prosecute or defend any action, civil or criminal, or to advise in relation thereto, no member of said partnership is permitted, then or afterward, to act as attorney or counselor on the side opposite to that upon which such original employment was had. This section does not prevent a person who has been employed as attorney and counselor in an action or proceeding before becoming a member of a partnership from appearing and acting therein until its completion.