Is a man's marriage contract to a Christian woman who is not committed and not chaste considered void, and is his intimacy with her considered adultery, and what is the duty of the first lawful wife in this situation?

A Muslim man is not permitted to marry a non-Muslim woman unless she is from the People of the Book (Jewish or Christian) and chaste, not an adulteress, based on the Almighty's saying: "And the chaste women from those who were given the Scripture before you." If either of these two conditions is absent, the marriage is invalid, and the man must separate from her. His intercourse with her would be considered adultery if he was aware of the prohibition. His first wife should advise him, and if he persists, she has the right to seek a divorce or khul' from him.