All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The guardian of a woman should be most concerned about choosing a good husband for the woman under his guardianship (the daughter in our case); a man who is religious and has a good moral conduct. Abu Hurayrah narrated that the Prophet said: “If a man whose religion and moral character are pleasing proposes to marry your daughter, then marry her off to him.” [At-Tirmithi]
The Prophet considered these two characteristics as a criteria for choosing a spouse. If the man whom you mentioned in the question deals with Riba (interest and/or usury) then he is not suitable to be a husband for your daughter.
Therefore, we advise you to be firm about this matter and eadeavour to convince your daughter to leave this man, and you should look for a pious man to marry her. It is religiously permissible for a man to look for a husband for a girl who is under his guardianship. The people who are better than us did so, and they were the companions of the Prophet like it was the case with ‘Umar as he proposed his daughter Hafsah to Abu Bakr and ‘Uthmaan .
However, if this matter is not facilitated, like if you did not find someone to marry your daughter, or that you found him but she refused to marry him, then we advise you to migrate to a Muslim country and then gradually drag your daughter to this. That is because a man does not have control over his daughter in those countries because of the law that is prevalent there. In addition to this, you will be taking her far from that man by migrating. For more benefit on migration from a non-Muslim country to a Muslim country, please refer to Fatwa 86405.
Also, you should supplicate Allaah as much as possible to guide her because the supplication of a parent (for his child) is accepted. Abu Hurayrah narrated that the Prophet said: "There are three supplications that will undoubtedly be answered (by Allaah): The supplication of a victim of injustice, the supplication of a traveler, and the supplication of a parent for his children." [Ibn Maajah]
On the other hand, if it is believed that deserting her will benefit her, then it is permissible to desert her but if it is feared that this will make her even more stubborn and corrupt, then it is more appropriate not to desert her. For more benefit, please refer to Fatwa 84976.
Finally, you should let her know that her marriage to the man whom you mentioned in the question or any other man is not valid without the consent of the guardian who is her father. In case you do not find a means to keep her away from that man, and you know that she is attached to him, then it is more appropriate to marry her off to him because it is better for a woman to marry a dissolute man rather than stay with him in an illegal relationship that may lead to illegal sexual intercourse.
Allaah Knows best.