So if i stop caring about someone who is willingly taking abuse of her partner and not walking away when she can, i become the bad guy?

no your not the bad guy, you just are not trained to work with people who have special needs, if she has not learned by now call the authorities on the abuser and say they are taking advantage of a RETARD. 🙄

If she isn't leaving, clearly the abuse is not a big deal to her. I would move on.