WHAT SHOULD A TEACHER DO IF...?

Your decision about students calling you at home is up to you, but I would be firm about that boundary line, even with this girl. In the US, what she is doing is illegal and I would report her to someone. I would probably contact my principal first. I might tell her that I had to report her situation to lessen the sense of having violated her trust, but I would not hesitate because she won't be able to come to me in future situations if she dies tonight. This is beyond the work that a teacher has to do and outside of a teacher's qualifications. Be there, open and receptive, and then get her the help that she needs and be happy you had the opportunity.