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.
You should begin by offering sincere advice to this girl, remind her of Allah, the consequences of sin, and the seriousness of her actions, especially if they have reached the point of Kufr (disbelief). If she listens and changes for the better, then all praise is due to Allah. However, if she remains unresponsive and you lose hope in her accepting your admonition, then it is best to cut ties with her, as continuing a relationship with her brings no benefit.
The Prophet, sallallaahu 'alayhi wa sallam, said, as reported by Abu Sa’id al-Khudri: “Do not keep company except with a believer, and do not serve your food except to a pious person.” [Abu Dawud, At-Tirmidhi, and Ibn Hibban]
In another Hadith, it is stated: “A person follows the religion of his close friend, so let each of you look at whom he befriends.” [Ahmad, Abu Dawud, and others]
Allah knows best.