The answer is rather complex. There are three legal concerns. First, was the evidence obtained illegally? If so, it might not be admissible given the "exclusionary rule". The second is whether the evidence is authentic; the third is whether it is hearsay.
I mainly address the legality issue, since it is the most complex. Two things could make obtaining such evidence illegal (which is possibly relevant to whether the evidence is admissible). One would be if it was obtained by gaining unauthorized access to a computer (in violation of the Stored Communications Act). Since the computers were not personal property of the employees and the accounts were not hacked into, there seems to be no violation of that law. The other way is that it might constitute “illegal recording” i.e. wiretapping. I will mostly cite Texas wiretapping laws, which are quite similar to federal laws.
The basic prohibition (16.02(b)) states
A person commits an offense if the person:
(1) intentionally intercepts, endeavors to intercept, or procures
another person to intercept or endeavor to intercept a wire, oral, or
electronic communication;
The word “intercept” is defined to mean
the aural or other acquisition of the contents of a wire, oral, or
electronic communication through the use of an electronic, mechanical, or other device.
Taking a screen shot of a message is acquisition of the content of a communication; it uses a device (a computer). So the message was intercepted, under a plain reading of the statute. (I will leave open the question of whether the “interception” has to be contemporaneous with the act of communicating: there is case law about this issue alluded to in United States v. Smith, 155 F.3d 1051, but TMI).
The nature of the communication is important. A Skype text (but not a Skype voice transmission) is an electronic communication, defined as
a transfer of signs, signals, writing, images, sounds, data, or
intelligence of any nature transmitted in whole or in part by a wire,
radio, electromagnetic, photoelectronic, or photo-optical system
A Skype text is not an oral communication. This distinction is important, because the definition of “oral communication” includes an escape clause that may allow recording. An “oral communication” is defined as:
an oral communication uttered by a person exhibiting an expectation
that the communication is not subject to interception under
circumstances justifying that expectation. The term does not include
an electronic communication.
(where in the second instance i.e. the start of the block quote, “oral communication” has its plain meaning). So even though the law generally forbids intercepting communication, in the case of oral communications, and only oral communications, the communication has to be made under circumstances where you would reasonably expect the communication to be private. By contrast, there is no such limit on what constitutes an electronic communication (nor a wire communication).
As noted in this article, “Wire communications are protected against interception regardless of the speaker’s expectation of privacy”, citing Briggs v. American Air Filter, 630 F.2d 414. That court reasoned that
Wire communications, unlike oral communications, are protected against
interception by electronic, mechanical, and other devices regardless
of the speaker's expectation of privacy. Compare 18 U.S.C. § 2510(1)
with 18 U.S.C. § 2510(2)
It is important to notice what 18 U.S.C. § 2510(1) actually says:
“wire communication” means any aural transfer made in whole or in part
through the use of facilities for the transmission of communications
by the aid of wire, cable, or other like connection between the point
of origin and the point of reception (including the use of such
connection in a switching station) furnished or operated by any person
engaged in providing or operating such facilities for the transmission
of interstate or foreign communications or communications affecting
interstate or foreign commerce.
The court’s conclusion that “wire communications, unlike oral communications, are protected against interception” doesn’t mean that there is an express statement within the law saying effectively “even if you don’t have a reasonable expectation of privacy, it’s still not okay to intercept”. Rather, that conclusion follows from the fact that there is a general prohibition against interception, plus there is no “expectation of privacy” escape attached to the definition of wire communication, as there is in the case of oral communication.
So there is some merit to the argument that the message was illegally intercepted. But now we come to the “so what” part. There is an “exclusionary rule”, that evidence obtained illegally can’t be used as evidence, but (quoting the article supra) “normal Texas law only prohibits the introduction of illegally-obtained evidence in criminal trials”: per Texas Code Crim. Proc. art. 38.23.
(a) No evidence obtained by an officer or other person in violation
of any provisions of the Constitution or laws of the State of Texas,
or of the Constitution or laws of the United States of America, shall
be admitted in evidence against the accused on the trial of any
criminal case.
You can use illegally-obtained evidence in civil trials (Baxter v. Tex. Dept. of Human Resources, 678 S.W.2d 265, 267):
The exclusionary rule codified in Tex.Code Cr.P. art. 38.23
(Supp.1984) is inapplicable to the present case because the Code of
Criminal Procedure applies only to criminal actions, and this is a
civil action arising under the Texas Family Code. Tex. Code Cr.P.Ann.
art. 1.02 (1977)
Although illegally-obtained evidence might admissible in a Texas civil trial, there is also an overriding exception at the federal level, whereby illegal evidence is excluded from all proceedings comes from the Wiretap Act, per 18 USC 2515. This states that intercepted wire or oral communications are broadly excluded:
Whenever any wire or oral communication has been intercepted, no part
of the contents of such communication and no evidence derived
therefrom may be received in evidence in any trial, hearing, or other
proceeding in or before any court, grand jury, department, officer,
agency, regulatory body, legislative committee, or other authority of
the United States, a State, or a political subdivision thereof if the
disclosure of that information would be in violation of this chapter.
If an oral or wire communication is intercepted, it cannot be used in a civil case (or in any other official action). But this provision does not apply to electronic communications. The reason for the omission of electronic communication from this law is that this particular section was enacted in 1968, when there was no such thing as electronic communication, but §2511 was amended subsequently.
So, the bottom line is, the evidence is probably admissible, if it doesn't fail on authenticity or hearsay grounds.
