M. R. Evid. 503: Waiver of privilege by voluntary disclosure.

(a) General rule. A person upon whom these rules confer a privilege against disclosure waives the privilege if the person or the person's predecessor while the holder of the privilege voluntarily discloses or consents to disclosure of any significant part of the privileged matter. This rule does not apply if the disclosure itself is privileged.

(b) Joint holders. Where two or more persons are joint holders of a privilege, a waiver of the right of a particular joint holder to claim the privilege does not affect the right of another joint holder to claim the privilege.