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I'll admit up front I don't know a whole lot about when privelege is waived.
My definition of waiver of privilege:
Difficult to assert and easy to waive.
From Lexis:
CA evidence code: A privilege . . . is waived with respect to a communication protected by the privilege if any holder of the privilege, without coercion, has disclosed a significant part of the communication or has consented to disclosure made by anyone. Consent to disclosure is manifested by any statement or other conduct of the holder of the privilege indicating consent to the disclosure, including failure to claim the privilege in any proceeding in which the holder has the legal standing and opportunity to claim the privilege.