Hmm...she sounds a bit like me :-(
That was what my boyfriend liked least about me. Opinionated, argumentative, aggressive.
Then he had me realise how much he disliked it. He would say so calmly what he was feeling. He told me that I was difficult to talk to when I would get in those moods. But what really stuck with me is when he described it to me once and said 'when you get like that I think "here we go again, she's never happy, I can't do anything right". That's when I realised that when i was yelling it really didn't have anything to do with him and it made me so upset to think he felt like he was doing something wrong. Now when we disagree I try to be civil about it. I don't raise my voice. I have a few times, but not nearly as much as I used to, and he's really noticed that. I like that he's appreciated it. Maybe you could try something similar? She's likely angry about something else in her life, like I was, and maybe she just needs a gentle wake up call for her to realise that you feel hopeless in the situation?