Yes i agree with chicklin, you MUST contact the social services, if only so that if anything DOES (hope not) happen in the future then at least it has been logged from the start, dates and times, etc......... is there no-one in the family that she IS still talking to, that could maybe still be able to call round (casually) and make sure that the child is ok? Most of all you mustnt give up, i have a feeling you will need to be there for her when this realtionship of your daughters finally and hopefully comes to an end!
Good luck, and let us know how you get on x