Praise be to Allah.
We put this question to Shaykh Muhammad ibn Saalih al-‘Uthaymeen, may Allaah preserve him, who answered as follows:
She should complete the ‘iddah of divorce.
Does she also have to observe the ‘iddah of one who has been widowed?
No, she does not have to do that, because the marital relationship ended with the final talaaq.
And Allaah knows best.