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.
Establishing an affectionate relationship between a man and a non-Mahram woman is an abominable sin and an act of Jaahiliyyah (i.e. the Pre-Islamic Era of ignorance) which Islam abrogated, and it is a reason for much evil and corruption. For more benefit, please refer to Fatwa 81356.
We hope that that girl has repented to Allaah a valid repentance because repentance is only valid if it meets some conditions; these are already clarified in Fatwa 86527.
Also, she should endeavor to protect herself from going back to such a relationship and observe Hijab and avoid mixing with non-Mahram men and keep company with pious women.
Moreover, what that young man is doing, i.e. blackmailing her and threatening to disclose the matter, is absolutely impermissible. So, he should be reminded to repent to Allaah from such a relationship. If he repents and stays steadfast on the obedience of Allaah, then she may think about whether or not to accept to marry him, and she should perform Istikhaarah prayer about him. If there is any good in her marrying him, then Allaah will facilitate their marriage, otherwise Allaah will drive him away from her and keep her safe from his plots. For more benefit on the Istikhaarah prayer about marriage, please refer to Fatwa 81434.
In case he does not repent or that he repented but she does not want to marry him in principle, and he keeps blackmailing her, then she may threaten him to take the matter to the authorities. If we presume that he carries out his threat, then she should be patient and first seek the help of Allaah, and then seek the help of some pious and righteous people to remind those who may blame her from her family or those who know her that repentance wipes out all previous sins and that some people who are better than her had committed such sins and then they repented to Allaah.
Finally, what happened to this girl should be a lesson for Muslim girls to be careful of evil people who want to play with their emotions and then lead them to Zina, and then these girls would regret when it is too late.
Allaah Knows best.