I need advice on my friend who hurts herself purposely?

There is this girl in my class we just started talking but she is pretty nice. my friend said in the past she was a bully which proved to be true but she is quite nice now. I may not have known her for long but i know the history too her she is in foster care. and today in class she asked for my elastic to keep snapping herself with i was freaked out and said no and thought she was joking at first then she continued and said " come on i promise i won't get blood on it" and she was serious. so i said no ok iam not letting you cut yourself! and she stopped then later continued saying she cut herself and that her roommate did too. then a few hours after she started talking again and said you know i consider cutting a sport. i may not know her that well but i care for her safety and don't want anyone to get hurt and she seems like a nice young girl.We remember no one try to same jokes.