Sorry you and your friends are going through this. Honestly though, there really isn't anything you can do. Denial is a very powerful force in an alcoholic relationship.
I am concerned about the sexual assault. I understand not wanting to report it, but their friend is not doing anyone any favors (including herself) by sweeping this under the rug.
Right now you really can't do anything except be there for your friend and her child. Maybe at some point she will be ready to reach out for help. Any pushing on your part will likely only result in resentment and more denial.
One thing I did with my axb's mother who is also an enabler was to slip some Alanon literature in with the handmade Valentines my son made for her. Don't know if it had any effect, but maybe if she gets desperate enough she'll check out a meeting. And it made me feel better.
Have you tried an Alanon meeting? That helped me get over the powerless/MUST ACT NOW feeling that used to plague my life. Plus you can pick up some literature to "accidentally" leave at your friend's house.