Laws of niddah for a Non-Jewish woman
Question
If a non-Jewish woman (whose father is a Jew) counts days from niddah, prepares for mikveh properly and goes to a spring-fed lake and recites the appropriate beracha, is she in niddah? Is a non-Jewish woman considered a niddah or non-niddah, just as a Jewish woman is?
Answer
The laws of niddah do not apply to Non-Jewish women. Menstruation does not render a Non-Jewess prohibited to her husband, and none of the laws related to ritual impurity and distancing from her husband are applicable to her. The laws of niddah are exclusively relevant to Jewish women.
It is evident that there is no prohibition for a gentile woman to observe the practice of counting seven clean days and immersing in a mikveh. However, it is important to note that there is no practical necessity for her to do so.
Source
Niddah
34a, Rambam, ‘The impurity of mishkav and moshav’ 2:10.