Praise be to Allah.
If a man divorced his wife irrevocably when he was healthy, then he died during her ‘iddah, she should complete the ‘iddah of divorce, not the ‘iddah of one whose husband has died. She does not inherit from him and she should not observe mourning for him.
For more details of that, please see the answer to question no. 111905. See also al-Mawsoo‘ah al-Fiqhiyyah, 29/325
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: If the husband dies, having divorced his wife irrevocably, during the time of the ‘iddah, should she observe mourning for him?
He replied: The correct view is that she should observe the ‘iddah of her divorce, not the ‘iddah of one whose husband has died, and she should not observe mourning for him.
If the divorce occurred during his final illness but he died, then she inherits from him, but she should not observe mourning for him, because her inheritance in this case is not because she was his wife, but because he wanted to deprive her of it, so he is to be punished by having his plan thwarted. Hence if she died during the ‘iddah, he does not inherit from her.
End quote from Thamaraat at-Tadween min Masaa’il Ibn ‘Uthaymeen (p. 116)
For more information on the types of ‘iddah for a divorced woman, please see the answer to question no. 12667
And Allah knows best.