"In McKenzie v. Lynch, 133 N.W. 490 (Mich. 1911), a husband settled a civil claim with a man who had engaged in "criminal conversation" with his wife. As part of the settlement, the husband promised not to "do anything whereby this matter will acquire any publicity whatever." Id. The court found this provision unenforceable because it could prevent the husband from "becom[ing] a complaining witness, in a criminal proceeding" against the other party to the contract. Id. at 491."
(Alan E. Garfield, "Promises of Silence: Contract Law and Freedom of Speech," Cornell L. Rev. 83, 261.)
That is to say that under these circumstances, one party could not sue the other for breaching a non-disclosure agreement by testifying as a witness in a criminal trial. I'm sure there's more recent and more relevant legal authority out there somewhere.
"In McKenzie v. Lynch, 133 N.W. 490 (Mich. 1911), a husband settled a civil claim with a man who had engaged in "criminal conversation" with his wife. As part of the settlement, the husband promised not to "do anything whereby this matter will acquire any publicity whatever." Id. The court found this provision unenforceable because it could prevent the husband from "becom[ing] a complaining witness, in a criminal proceeding" against the other party to the contract. Id. at 491."
(Alan E. Garfield, "Promises of Silence: Contract Law and Freedom of Speech," Cornell L. Rev. 83, 261.)
That is to say that under these circumstances, one party could not sue the other for breaching a non-disclosure agreement by testifying as a witness in a criminal trial. I'm sure there's more recent and more relevant legal authority out there somewhere.