This question is purely for academic purposes, and for clarification. It is not chas veshalom nogea lamashah since jews are an am kadosh. Does a Jewish man commit adultery with a married non jewish woman? Is a non jewish woman a niddah?
A non-Jewish woman is not considered a niddah. The laws of niddah were given to Jews alone, and not to non-Jews.
Non-Jews are considered married to one another, with regard to a non-Jews who commits adultery with a non-Jewish married woman (Rambam, Melachim 9:7; Maggid Mishnah, Ishus 1:1). However, with regard to a Jew, all non-Jewish women have the same (prohibited) status, and there is no distinction between a married or unmarried non-Jewish woman.
Therefore, it is not strictly adultery, however, it remains a very severe Torah prohibition (see Rambam, Issurei Bi’ah 12:4, 13).