As I understand it, the halacha interprets "lo tina'af" as prohibiting sex between a man (married or unmarried) with a married woman not his wife. Is a gentile woman, in a marriage between gentiles, considered "married" for the purposes of "lo tina'af"?
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Rambam » Sefer Shoftim » Melachim uMilchamot » Halacha 7
So once she had relations with her husband she is considered to be married and a adulteress relationship is forbidden.
In Sanhedrin 58b Rav Dimi brings a case of a master who designates a maidservent (gentile) to a male servant (gentile) and another gentile comes along and has biah with her he is killed.
Rav Nachmun writes when is the maidservant considered set aside to him when she is called ploni's girl. See the gemara starting from daf 57 on the topic of bnei noach.