Answer
(Fatwa: 938/858=L/1429)
The girl should repent and seek forgiveness from what she has committed, for she had unlawfully been with that Christian boy for so long. Whenever she had intercourse with him it was adultery and the children born to them were illegitimate. Now when the boy has accepted Islam, if the guardians of the girl want to get her married to him, then they can do this but the two must observe purdah (veil), they should live separately and avoid seeing each other until they marry.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband