What is the ruling on a man talking to an engaged woman and seeking to have her divorced from her fiancé so that he can marry her, and is this considered marital betrayal, and what is the sin of both parties, and how is it expiated?

A young man calling a foreign girl and instigating her to leave her husband so that he can marry her is haram (definition) (forbidden), a great evil, and a betrayal of trust. Indeed, inciting a woman against her husband is among the major sins. Some scholars have even gone so far as to say that the marriage of a woman to the one who instigated her against her husband is invalid, as a punishment for him. It is obligatory upon the young man and the girl to cut off this relationship immediately and hasten to repent to Allah Almighty. If the girl dislikes her husband, she has the right to seek khul' (divorce by mutual consent, initiated by the wife), and her guardian has no right to force her to marry against her will.