Shalom and thank you for your question! You wish to know the Torah approach when a person is faced with a terrible dilemma of whether or not to commit a grave sin in order to save somebody or something...
The approach of the Torah in such a situation is to stand firmly on one's principles and not give in. Why? When we are talking about 'violating marital vows' we are talking about the sin of adultery, which is spoken of in Exodus 20:12, "Thou shalt not commit adultery..." and it is the seventh of the Ten Commandments. (By the way, there are six hundred and thirteen commandments in the Torah, but the Ten Commandments could perhaps be viewed as something akin to chapter headings, since there are many other commandments and many details to learn.)
Even though 'Mrs. Crachet' is in a very challenging situation, she is nevertheless forbidden to give in to her pursuer.
Not giving in will take a lot of courage and faith. It is a test, and this is actually what we are here in this world for. I will share a story with you about a great and pious Torah scholar called Rabbi Meir of Rothenberg. Rabbi Meir (known by the acronym Maharam,) lived approximately from the 1220's till about 1290, in Germany. That period was also a period of terrible persecution against the Jews, and he tried valiantly to help his brethren. However, while he was on his way to try to make aliya to the Holy Land- Israel, an antisemite recognized him, captured him, and took him to a fortress to be imprisoned and held up for ransom. The Jewish people would have been willing to make tremendous sacrifice in order to ransom him, a fact that was not hidden from the antisemitic rulers. That is precisely what they wanted. However, Rabbi Meir would have none of it. He knew that if he would be ransomed, the rulers would continue to arrest Rabbis and hold them up for ransom. In order not to be a burden to his people, Rabbi Meir selflessly remained in prison for several years until his passing. (He was able to study and write while he was there.) Even after that the 'rulers' did not surrender his body for burial for another few years, until a good, wealthy Jew, paid ransom for the body and was then buried next to Rabbi Meir. This is an example of passing the test of faith even at one's personal expense. Of course, he himself suffered - he did not inflict damage on someone else. However he WAS separated from his family, from whom he had been so rudely snatched away. Sometimes we do do not understand what is decreed from above, but we can all identify with Rabbi Meir's self sacrifice for his fellow, and for ideals and principles.
This brings us to another part of your theoretical scenario. Faith and trust. Marriage is a G-dly institution. If someone violates its vows, they are transgressing G-d's rules. We are required to trust that everything that happens to us is by Divine Providence. If keeping G-d's rules sometimes entails inconvenience or worse, we must believe that G-d has His reasons and it is for the good. (There are situations where we are allowed to disregard laws of Shabbat or the kosher laws, in situations of critical health needs, to save life, but this is not our case here.)
How do we trust in G-d? We live in a world chock full of media and multi media, with an absolute plethora of ideas and ideals, some positive and some - much less positive. When we expose ourselves to learning Torah, which expresses the Divine will and wisdom, we are tuning in to spiritual energy which can strengthen our resolve to do the right thing. Moreover, learning Torah and trying to practice its precepts draws down blessings from the spiritual realms to this earthly material world. (Non-Jewish people can study and practice the Seven Noahide Laws.) So when we are tested, like Mrs. Crachet, we can draw on our spiritual reserves of faith and trust in G-d, and at times this will cause us to merit immediate or eventual salvation.
There are many many stories of people who were saved from difficult situations due to their simple faith, or their connection with a holy spiritual leader. During the Vietnam war for example, there was a Jewish soldier, who had visited the Lubavitcher Rebbe, Rabbi Menachem Mendel Shneerson in New York, before being dispatched to Vietnam. The Rebbe instructed him to always be careful to perform the Jewish ritual hand washing before eating bread. As you can imagine, this is not always easy in a war situation. Jewish law even allows ways to get around this requirement in difficult situations. Our soldier found himself in a situation where there was no water available in the camp, so he slipped out of the camp and found a well in an abandoned village. There he drank and washed his hands in the required manner for eating bread. When he returned to the camp - no-one had been left alive. There was smoke. It had been shelled. Keeping the Rebbe's instruction had saved his life.
We hope that you never encounter such a challenge as Mrs. Crachet, but that you stock up on the spiritual connections mentioned above, just in case!
