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.
According to the consensus of the scholars it is not permissible for this girl to marry this Christian man. So, the relation between her and this man is pure Zina (fornication and adultery). Allaah Says (what means): {...they are not lawful [wives] for them, nor are they lawful [husbands] for them...} [Quran 60:10]
Therefore, every Muslim must reject this behaviour of hers and should hate her for Allaah's Sake until she repents. On the other hand, Muslims should take her case to anyone who could separate between her and this man.
For more benefit, please refer to Fatwa 84456.
Allaah Knows best.