A Christian cannot legally prohibit another believer from disclosing matters of judgment and justice without violating the ninth commandment.
The language of the ninth commandment is simple: “Thou shalt not bear false witness against thy neighbor” Exodus 20:16. The Westminster Longer Catechism helpfully expounds on the duties required and forbidden by the ninth commandment.
First, the ninth commandment prohibits churches and ministries from binding a believer from “freely, clearly, and fully” speaking the truth “in matters of judgment and justice” (WLC 144). The ninth commandment also prohibits believers from “concealing the truth” (WLC145). Obligating a witness or victim under a Non-Disclosure Agreement is one way to conceal the truth.
Obligating another believer to “undue silence in a just cause” is also a violation of the ninth commandment. If justice requires disclosure, a Christian must be free to speak. Binding the believer under an NDA is morally equivalent to refusing to testify yourself. If we are guilty of “holding our peace when iniquity calls for either a reproof from ourselves, or complaint to others,” then God’s law condemns us. Using legal means to obligate others to violate God’s law is no less of an offense, because for the one who “causes one of these little ones who believe in Me to sin, it is better for him that a heavy millstone be hung around his neck, and that he be drowned in the depths of the sea” (Matthew 18:6).
Second, a Christian victim has duties to God that mitigate against a need to prohibit disclosing misconduct. A Christian victim already has to balance their obligations to God. On the one hand, he or she has a duty of “sorrowing for and covering of their [neighbor’s] infirmities” (WLC144). This is consistent with 1 Peter 4:8 that “love covers a multitude of sins.” But this responsibility to cover is balanced by a duty on the other hand to make public accusations when the neighbor’s iniquity or misconduct calls for reproof or complaint to others (WCL145). Matthew 18 likewise obligates believers to make accusations public when the accused “will not listen” to the accuser.
Binding a Christian to a Non-Disclosure Agreement could put them in a position where the believer must choose between keeping a promise to man or obeying God. The believer who sins against a fellow Christian should take comfort in God’s grace and the duties that his brother in Christ has to cover his infirmities. To obligate a victim or a witness to silence regarding misconduct would be to slip down a path of destruction from sin to sin.