From the practical side, pursue Social Services. Your mother will have to persuade them that this is an urgent case.
The
Children's Legal Centre is a useful resouce to help understand her rights.
Their leaflet on
Child Protection may be helpful.
From the personal side, your sister's friend probably needs someone to talk to. Whatever the rights and wrongs it must be terrifying to think about getting involved with the Police, social services and leaving the only home she's ever known.
It may be helpful if your mother can give the girl the opportunity to speak to Childline 0800 1111.
Hope that helps.