This may sound drastic but if you are really at your wits end, sometimes the thing to do with mom's who stay in abusive relationships is to call CPS (CFS in my state).
It sounds harsh but it isn't until the threat of CPS in your daughters life which will make her see the light with this guy. Sometimes CPS can hook the moms up with resources and help that the average person cannot. They won't remove your granddaughter but they will investigate and spell it out for your daughter in very clear terms about what she has to do to protect herself and her child.
As a mom your daughter has to protect her daughter. In the eyes of CPS, a mom who goes back to the known abuser is child endangerment and they do get involved. A child who witnesses child abuse is experiencing a form of emotional abuse as well.
It sounds very harsh but I work with some very troubled moms and it is the last resort but often the ONLY thing which gets them to see the light that these men they pick to stay with are bad news all around.