It's never good to threaten people, and besides, it's not your place to tell her mother anything. Instead of that, why not just tell her that you can't be her sounding board anymore. She has a right to stay with him if that's what she wants to do and he has a right to drink if that's what he wants to do. Both of these people are adults and are out of your control. Once you remove yourself from the situation, it shouldn't matter to you one way or the other what they do. If it still does, then you have controlling issues that need to be dealt with.