A Jewish woman was in a relationship with a non-Jewish guy some years ago. She has a child from this relation. They finally broke up. Now that woman is in a new relationship with a Jewish guy, who happens to be a Cohen. They want to marry, but they have concerns that the orthodox rabbi will not officiate the wedding becuase she was involved with a non-Jewish guy previously. 1) According to Halachah, could she be considered Zonah? 2) Marrying that woman, would the guy become a Cohen Pasul? 3) Which sources would you recomend me to study concerning this issue?

A Jewish woman that had relations with a non Jew (amongst others) is defined as a Zonah. A Cohen is not allowed to marry a Zonah. If a Cohen does marry a Zonah – his child becomes a Chalal i.e. Cohen Pasul but he remains a Cohen. See more: Yevamot 61a&b; Shulchan Aruch Even HaEzer 6, 8; Kitzur Shulchan Aruch 145, 12