I have to agree with Jeff. There's very little you really can do.
You might want to also talk to the childrens teacher and see what advice
s/he has to offer, and maybe the teacher can then either talk to the
mother and suggest to her she get help or call the child protection
services or whatever they're called where you are.
If I was you I'd be very careful to not upset that family even further,
the husband is clearly violent and personally I would be scared that he
might come to my house if I seemed responsible if some government agency
stepped in.

I agree that the girl does seem to be testing your daughters friendship,
but that doesn't help you or your child, and as Jeff said, it's not your
job to provide councelling.

take care
nicole