Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
If the matter is as you mentioned in your question that the person is engaged only and the marriage contract has not been established it is enough to inform the fiancé that the person or her family is not interested in marriage to him. Engagement is not more than a promise of marriage.
If the marriage contract is completed then a wife cannot divorce her husband, but she may ask for a divorce for a sound reason.
If he denies her a divorce she may sue him in an Islamic court and they will look into the matter and decide what is lawful in Shariah.
Allah knows best.