I just read a book that might be helpful and healing to you, "why does he do that" by lundy bancroft. It really helped me see alcoholism and abuse are two different issues, not always combined, and that an abusive person will still be an abusive person even if she/he stops drinking.
It talked about how they fake a nice exterior so they don't look like abusers to anyone but the ones they abuse - the mask never slips.
I wonder if you are feeling upset because this is one more instance where you know the real abusive person and yet she gets to have this mask on in public that makes her look so good to others? And then others don't believe you, they believe her?
I was wondering that because that's kind of what has been going on in my family, my sister especially is good at putting on a fake front and appearing non-abusive so people don't believe me about the abuse unless they've witnessed it themselves. It makes me upset to see that fake front because it invalidates my knowledge of who that person really is.