There is principally no difference in Iddat of a divorcee and a widow except for the fact that the iddat of a widow is four months and ten days. In contrast to the iddat of a divorced lady, the iddat of a widow has been extended by forty days. The reason is that while a husband has been asked to divorce his wife in the period of purity in which he has had no sexual intercourse with her, obviously no such requirement can be proposed in the case of a widow. Since the basic purpose of iddah is to determine whether a lady is pregnant or not, therefore in order to exercise care, additional forty days have been added by the Quran to the waiting period of a woman who loses her husband.
Since in this partcular case as asked in the question, the procedure for a normal divorce has been observed before the death of the husband (i.e. divorce given in the period of purity), therefore the iddat period of normal divorce will be applied in this case (i.e. three menstrual cycles).
The Directive of Iddat:
Directives Relating To Widows:
Issues Regarding Iddah/Iddat: