Happy Wanderer and Joshuaace bring up a legitimate point. Traditionally, common law has allowed for evidence to be admitted in civil cases without regard for the manner in which it was obtained. Unlawful acts used to obtain the evidence was not considered in judicial decisions.
And Happy Wanderer and Joshuaace are correct that the exclusionary rule is generally limited to government actors and has generally not been considered applicable to civil actions or evidence obtained by private persons.
However, there are state statutes which include exclusionary provisions and there are constitutional principles to guide a court, and there is an ever-growing body of case law that supports exclusion or suppression of illegally obtained evidence in civil cases.
In 1928, in Olmstead v United States, Justice Brandeis questioned the use of illegally obtained evidence in court. It should be excluded, he said, to "maintain respect for the law; in order to promote confidence in the administration of justice; in order to preserve the judicial process from contamination."
And in Burdeau v McDowell, which defined the role of government actors under the exclusionary rule, the Court said that "judicial integrity" was harmed by the admitting of evidence illegally obtained. "To sanction such proceedings would be to affirm by judicial decision a manifest neglect, if not an open defiance of the prohibitions of the Constitution, intended for the protection of the people against such unauthorized action."
It was with Mapp v Ohio in 1961 that the exclusionary rule was extended to state criminal prosecutions and illegally obtained evidence, again with the mention of "judicial integrity." Mapp was follwed by People v Owens and, in 2000's Dickerson v United States the court said that evidence could be excluded as a matter of supervisory power.
In McNabb v United States, the court held that evidence obtained illegally should not be allowed, as it violated the "fundamental principles of liberty and justice," and the court based their decision in that case not on constitutional grounds but on the supervisory power of the court. Elkins v United States likewise used "supervisory powers of the court" to exclude evidence.
Common law allowed the introduction of illegally obtained evidence for purposes of divorce actions when adultery was a crime and the only means to dissolve a marriage was to prove adultery, and generally the only way to prove adultery was to obtain evidence through illegal means. This is no longer the case. In response to the changing divorce laws in the 1960s, several courts applied the exclusionary rule in their divorce actions, excluding any evidence obtained by illegal means. (for examples, see Del Presto v Del Presto, New Jersey, 1966; Williams v Williams, Ohio, 1966; Sackler v Sackler, New York, 1962).
From the dissent in Sackler: "It is a strange concept which would permit a court of law to encourage the commission of illegal acts by honoring the fruits of the illegality, and which would permit the perpetrator to win a lawsuit by deliberately violating the law."
Courts, in other words, have long recognized that judicial integrity is compromised with the introduction of illegally obtained material into evidence. With the availability of civil discovery where material can be obtained through legal means, and with no-fault divorces, there is less need for the admittance of such evidence.
As Ruth Bader Ginsburg said (quoting Elkins): "Nothing can destroy a government more quickly than its failure to observe its own laws, or worse, its disregard of the charter of its own existence."