Would the unmarried Muslim in this situation have committed Zina or Adultry?
If the answer is Zina, is it because the Muslim is unmarried or is it because the non-Muslim’s marriage is not considered Islamically valid?
The Muslim has committed fornication because (s)he is unmarried. The non-Muslim has committed adultery because (s)he is married. Both adultery and fornication are called Zina. Non-Muslim's marriages are considered valid.