All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
We congratulate your friends and this girl for the blessing Allah has bestowed upon them of entering Islam. We advise them to strive for everything that will help them to remain steadfast in it until death, such as learning Islamic knowledge, doing good deeds, keeping company with righteous people, and attending righteous gatherings.
If this relationship between him and the girl before their conversion to Islam was, according to their customs, a type of marriage, then this boy is attributed to him. Likewise, if they believe intercourse was permissible because he was her master, then his lineage is attributed to this man.
Islam demolishes what preceded it and erases the traces of adultery that preceded it. There is no harm in him marrying her with her guardian's permission if one of her guardians is a Muslim. Otherwise, the matter of her marriage will be handled by the Islamic bodies specialized in looking into Muslim issues. She can appoint a Muslim man known for being just to marry her if there are none.
Allah knows best.