It is marriage fraud when one marries an alien for the sole purpose of getting them lawful permanent residence in the U.S. An alien who commits marriage fraud is inadmissable to the U.S. for life. In the situation you describe, the couple appears to have had a real relationship that has gone bad, and that is not marriage fraud. A U.S. citizen who has started the process of gaining their spouse a green card may stop the process by withdrawing their support any time prior to the issuance of the green card.
In this case, if the U.S. citizen does not want the process to go along any further, she should just send a letter stating that to the nearest CIS office, with all the information she knows about the case, i.e. husband's name, birth date, alien number. That shoudl serve to stop the process. Or the US citizen may take the easy way out, and simply fail to appear at any future scheduled interview -- that will stop the case as well.
Both parties are subject to INA §275(c), 8 U.S.C. §1325(c)--Imprisonment for up to 5 years and $250,000 fine. To convict a person under this statute the government generally must prove: (1) the person knowingly entered into a marriage; (2) the marriage was entered into for the purpose of evading a provision of the immigration laws; and (3) the person knew or had reason to know of the immigration laws.
Whether or not they are prosecuted will depend on the U.S. Attorney's office in their district of residence.