Praise be to Allah.
If the situation is as you describe in the question, and the marriage contract had not been done between them, with the proposal from the woman's guardian and acceptance from the husband, whilst fulfilling all the conditions of marriage, with no impediments, then the woman mentioned does not inherit and she does not have to observe ‘iddah or mourning, because she is not a wife to her fiancé; rather she is still a stranger or non-mahram to him, because the marriage contract had not been done; rather all that happened was engagement and agreement with her relatives on the mahr only. This on its own is not regarded as a marriage, and there is no difference of opinion among the scholars (may Allah have mercy on them) concerning that. And if the family of the fiancée had taken any money from him, they have to give it back to his heirs. End quote.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him)