(This answer, necessarily brief and sketchy, is based upon the CCAR responsum no. 5757.1, available in Reform Responsa for the Twenty-First Century (New York: CCAR Press, 2010), vol. 1, pp. 347-356. See that responsum for a fuller discussion, along with citation of halakhic sources.)
The victim of sexual harassment may – or may not - have valid reasons not to report that harassment to the police. But Jewish law is not one of those reasons. To answer your question precisely, Jewish law does not “trump” civil law in criminal matters. When a Jew commits a crime, there is no halakhic prohibition against reporting that act to the authorities, especially in a democratic society where Jews enjoy equality and the full rights of citizenship.
The relevant principle here is dina d’malkhuta dina, “the law of the state is valid law.” According to that principle, which occurs several times in the Talmud, Jewish law recognizes the halakhic validity of a wide range of actions taken by non-Jewish legal authorities in the sphere of civil, criminal, and monetary law. Now this recognition has its limits; not every act of non-Jewish authorities is accepted as valid under the halakhah. Our tradition does not accept the right of the civil government to intervene in matters that pertain exclusively to matter os isur v’heter, Jewish ritual law. And even in non-ritual (“civil”) matters, the halakhah accepts the validity of the law of the state – dina d’malkhuta – only when it is truly “law,” that is, when it falls within the confines of legitimate state authority. State authority is “legitimate” when it serves ends that are traditionally recognized as part of government’s proper function: for example, laws that levy taxes, that provide for the public welfare, that regulate property, contracts, and business activity, and so forth. So long as these laws apply equally to all citizens and draw no unfair distinctions among them, they are regarded as valid under the halakhah, simply because political communities are entitled to legislate in these areas.
One of the spheres of legitimate governmental authority is that of criminal conduct. All states are empowered – indeed, they are obliged – to protect their people against violations of their persons and property. Laws that define and punish the offense of sexual harassment clearly and obviously come under this rubric. They are proper expressions of legitimate state authority, and for that reason they are regarded as valid under Jewish law.
One occasionally will hear, in objection to the above, the argument that Jewish law prohibits us from surrendering a fellow Jew to the jurisdiction of Gentile courts. Such a prohibition, if it exists in theory, cannot seriously be said to apply to the legal system of a state in which we Jews enjoy equal rights and full citizenship. The law courts of such a country are not “Gentile” courts; they are our courts, belonging to us no less than to anyone else. When we freely accept this citizenship and participate equally with our fellow citizens in the making of the laws, we can hardly claim that we are somehow exempt from the enforcement of those laws or that they apply to everybody but the Jews. Such a claim is absurd, even outrageous, to the point that it constitutes a chilul hashem, a profanation of God’s holy name. So long as it is legitimate in its authority and equitable in its application, the laws of the state – including the laws concerning sexual harassment – are obligatory upon us just as they are obligatory upon all the other citizens of the state.