Is supplicating for marriage to a specific person, after they have committed a sin, considered a sin? And what is supplicating for sin?

The default rule is the permissibility of supplicating for lawful matters. Marriage to a disobedient person is permissible, so there is no harm in supplicating for marriage to a specific person, even if they are disobedient. However, it is not appropriate to supplicate for marriage to a transgressor (one who commits major sins or persistently commits minor sins) unless he repents, due to the fear of his transgression's danger to the woman's faith and her children. It is better to supplicate for his guidance and righteousness, and if his affairs become righteous, she should seek Allah's guidance concerning marrying him. As for supplicating for sin, it is supplicating for something that Allah the Almighty has forbidden.