B might be subject to prosecution for second-degree extortion, in Washington state. RCW 9A.56.110 defines "extortion" as
knowingly to obtain or attempt to obtain by threat property or
services of the owner, and specifically includes sexual favors
and second-degree extortion relates to the nature of the threat – it includes "To reveal any information sought to be concealed by the person threatened". However, the prosecution would have to establish that B attempted to obtain something from A, which would presumably be the continuation of the relationship.
Under the comment-modified scenario where the spouse knows of the relationship, the threat would be to reveal whatever the criminal activity is that the A's are engaged in. In that case, there is a potential defense under RCW 9A.56.130(2):
In any prosecution under this section based on a threat to accuse any
person of a crime or cause criminal charges to be instituted against
any person, it is a defense that the actor reasonably believed the
threatened criminal charge to be true and that his or her sole purpose
was to compel or induce the person threatened to take reasonable
action to make good the wrong which was the subject of such threatened
The question then would be, how believable it would be that B's action was solely an attempt to get A to stop their criminal behavior? There is a path leading to a criminal conviction, but whether B is taken down that path depends on a lot of facts (and impressions) to be established at trial. Since A's spouse seems to know about the relationship and is unaffected by the information, giving over the thumb drive would not have any of the consequences outlined under the definition of "threat" in the law, such as revealing secrets, or harming a relationship.
"Threat" is defined (RCW 9A.04.110(27)) as "to communicate, directly or indirectly the intent to..." (list then follows, including "do any other act which is intended to harm substantially the person threatened or another with respect to his or her health, safety, business, financial condition, or personal relationships"). That means that a threat is communicating an intent to do something: it is not doing the thing. In order to constitute a threat against A, B would have to communicate to A the intent to reveal, rather than just directly revealing. If we assume that that did not happen, the act would not fit the description of a threat.
There is also Ch. 9.73, which includes the "wiretapping" law. (RCW 9.73.030, labeled "Intercepting, recording, or divulging private communication"). Washington is an all-party consent state meaning that all parties have to consent to a recording. Since email is an electronic means of communication and that is all that is required to be within the scope of the law, it might seem that this could be a violation of the wiretapping law. Nevertheless, the law says that
Except as otherwise provided in this chapter, it shall be unlawful for
any individual, partnership, corporation, association, or the state of
Washington, its agencies, and political subdivisions to intercept, or
and notice that the word "divulge" is not included in the law. Both parties clearly know that the conversation is being "recorded". The section does eventually include "divulging" as a permitted (not prohibited) acts:
(4) An employee of any regularly published newspaper, magazine, wire
service, radio station, or television station acting in the course of
bona fide news gathering duties on a full-time or contractual or
part-time basis, shall be deemed to have consent to record and divulge
communications or conversations otherwise prohibited by this chapter
if the consent is expressly given or if the recording or transmitting
device is readily apparent or obvious to the speakers.
The section has a number of places where it is specified that one can legally divulge a communication, and nowhere does it prohibit divulging information (recording and intercepting are prohibited, but not divulging). Consequently, those parts of the statutes are without effect. In such a case, the courts are likely to re-interpret the wording of the statute so that it is sensible to talk about "divulging" (the court always find an intent underlying the words of a legislative body). It might then turn out that the court would find that the statutes also prohibit divulging without consent. One local attorney claims that the law "punishes you if you publish the recording", without explaining the authority for that conclusion (anyhow, passing on a copy of the emails is not "publishing").