Is the fiancé's betrayal of his fiancée through an abominable act (fahisha) like a husband's betrayal of his wife, and does a dwelling lose its blessings if a sin is committed therein?

This man is a stranger to you, and engagement is merely a promise of marriage. Therefore, it is not permissible for you to speak with him, or to be alone with him, or to allow him to look at you. Adultery is a grave sin, one of the major misdeeds, and there is no difference in its prohibition between a married person, an engaged person, or anyone else. However, the punishment for a married person is more severe. Allah Almighty says: "And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way." If this man repents sincerely, then there is no impediment to marrying him or to residing with him in the same apartment.