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.
What happened to this girl indicates the danger of an illicit relationship between a man and a woman, showing how such connections can become a gateway for Satan and result in immoral behaviour. Allah the Almighty says: {O you who believe, do not follow the footsteps of Satan; whoever follows the footsteps of Satan, he only enjoins indecency and evil acts...} [Quran 24: 21]
Thus, she must repent to Allah with sincere repentance and completely cut off any relationship with this man.
As for the child, he is not attributed to the adulterer, but rather to the mother. She is responsible for him, and it is not permissible for her to hand him over to this non-Muslim man.
It is also not permissible to restore the hymen if its loss was due to fornication, as this may encourage immoral behaviour, involves exposure of the major private parts, and includes other religious prohibitions.
Lastly, if this man embraces Islam, adheres to the commands of Allah, demonstrates sincerity in doing so, and the woman repents sincerely, then there is no objection to them getting married.
Allah knows best.