She has always been pretty opinionated (especially in areas such as politics, child rearing etc). She genuinely thinks that her way is the only right way.
She has one friend specifically who has quite different opinions on the topics above, however having been friends for thirty odd years they obviously have other things in common etc.
Lately, maybe the last year or two, DMs general tolerance for others has reduced, her judgy pants are generally hoiked a bit higher and she has adopted quite a smug attitude towards anyone that does things differently to her.
Sober, she is tolerable and, in the main, a nice person.
When she is drunk, she is a PITA and tonight I witnessed a frankly disgusting personal attack on the Friend I mentioned at the top. They had both had wine, Friend had had just under a bottle of wine, DM had had nearly three. I was sober having popped in on passing quite late into the evening.
DM was incredibly rude to Friend every time she spoke about anything and thinking it was down to the volume of alcohol I busied myself in changing the conversation each time it soured. I was shocked at (1) quite how brutal she waa being and (2) that Friend was simply accepting being spoken to like that.
When my DM stepped out of the room I asked Friend if she was ok and she said "no but I am trying" and her eyes were fillig up. I offered her a lift home whenever she needed it.
Evebtually DMs attitude got so bad that Friend said it was time to go. I dropped her home and said I hoped she was OK and I was disgusted at the way she had been spoken to. She then told me DM was like that the whole night, and is even like that when she visits Friend in her own home. Friend says she feels picked on for having different opinions, different politics and that DM focusses on them abd basically belittles her all the time.
DM has grown up children now, and Friends child is still primary age, and DM expects Friend to be as avaliable and social as she herself is, and fails to factor in that Friend is working ft, has a child and is a lone parent.
Sorry this is so long 
As DM lives right next to me, I popped back in on my way home to see if her other friend needed running home (walking the two streets is an option but I was free and thought itd be nice). DM attacks me verbally as soon as I walk back in - what the hell is the matter with you blah blah. I said In here to offer Other Friend a lift home. Long and short of it I said how shocked I was at the treatment of her Friend and that Ive never seen anyone speak so badly to someone they are close to. I said Id not go into it tonight but maybe she should go see Friend when she sobers up tomorrow and has a chat
I was told repeatedly "just go away". So I did.
I feel so so angry and disgusted at how nasty she was to her friend tonight. And of the revelation that she has been like this with her for some time.
All I can think to do is talk to my dad about it.....but what do I say!? Imo she needs to have a sober talk with the friend. She needs to maybe address her drinking. She turns truly awful when she has a drink, even more so lately.
I want to know if he has seen a change in her the last year or so. I know I have and I dont even live with her.
Thank you for reading so far and if anyone can help with what they think my next step should be, and offer any advice.