What a heartbreaking situation for this young girl. How frustrating it must be for you and her dad.
I wonder if her school teachers, counselors and other personnel see the signs. Maybe ya'll could start working with them to see about helping her. If her physical appearance is affected the teachers may pick up on that and if her dad approaches them that he has concerns - they may be willing to help or voice their concerns with him.
I know that he would have to approach it cautiously as you don't want to be accused of slander by saying the mother is an alcoholic - but maybe he could say it appears she has some issues that are affecting her ability to provide all the care that the daughter needs.
I agree with the suggestions on seeking Legal council from someone in your area that may know how to help - possibly attending some Al-Anon meetings or even OPEN AA meetings in your town may share some experience, strength and hope to give you and your partner some direction.
PINK HUGS,
Rita