What should a woman do to atone for her vow if she vowed to divorce her future husband's first wife?

This vow is not permissible to fulfill, due to the corruption and harm it entails for a Muslim. It is an unlawful vow of disobedience, and fulfilling it is forbidden. Most scholars hold that there is nothing due for it, while some have said that it requires an expiation for breaking an oath. It is more prudent for your colleague to offer an expiation for breaking an oath if she marries that man, which is to feed ten poor people, or clothe them, or free a slave. If she cannot find these, then she must fast for three days.