Praise be to Allaah.
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)