All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
If the husband of this woman separated from her with a divorce, then her waiting period is the same as that of any divorced woman. Ibn Qudaamah said: 'The waiting period of the separation between the married partners (regardless of the method via which their relation terminated) is the same as the waiting period of divorce…according to the view of the majority of the scholars.'
For more benefit on the kinds of waiting periods of a divorced woman, please refer to Fatwa 82929.
A woman whose waiting period is calculated by months, like a young girl and a woman who passed the age of menopause, her waiting period is three months, and we did not come across any statement of the scholars who said that her waiting period is one month.
Nonetheless, some scholars are of the view that one monthly menses is enough to make sure that an adulterous (or fornicator) woman is not pregnant.
Allaah Knows best.