If unmarried Muslim has sex with married non-Muslim, is this considered Fornication or Adultery

Question

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?

Answer 1

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.