I agree with those who advised contacting Social Services who I would assume can help liaise with the police as appropriate.
If she is worried about the threats then she should address this with them, I'm sure they could make their own (ie not "direct" from your friend) approach based on the fact she is missing school and with a parent with a violent past who does not have legal custody - I'm hoping my faith in Social Services is not over enthusiastic here.
They would have to act in the best interests of the child. They should also be able to advise on potential options for her if she feels threatened.
If they get involved it may give her a chance to speak to responsible adults and, at least, get her back into school where there are other adults she can speak to if she has been threatened at all, is unhappy etc...