Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Dear sister, it is an obligation upon you to remind this girl about the Punishment and Anger of Allah, and inform her that what she is doing is very serious. If she repents and gives up what she is doing then this is what is required. Otherwise, threaten her that you will inform her family about that. If she gives up, it is all right otherwise inform her mother about what she is doing, or inform her that she did not come to see you for a given period of time. Informing her mother is considered as helping in virtue, righteousness and piety, not helping in sin and transgression.
If you keep quiet about what she is doing, you are helping her in her sin.
May Allah enable everyone to do what Allah loves and what pleases him.
Allah knows best.