Niece has lying problem and just left home

Long story short, my brother had a few rules for his daughters and one daughter has constantly pushing the boundaries. I know teenagers go through rebellious stages, but she is beyond help. She is a compulsive liar, meets random people on the internet, told stories about her dad being is prison, etc. Recently, my brother has reached out to see if I can help, to talk to her. I know she is hurting, she has a broken family, and she is extremely insecure, looking for validation from whoever will give it. I tried to talk to her and reason with her, she wants to move out with her boyfriend (another broken rule) and she has run away from home. Any suggestions on what I can do? She has been lying to me saying she is with friends but she is with her boyfriend. That’s one of the many lies she has told to me... any suggestions on how I can help?