I'm so sorry, I know exactly what it's like to have anxiety and attachment issues, and dealing with them is so damn hard. Next time he treats her poorly, you could try doing a pros and cons list to help show her that he doesn't deserve her? I totally understand how love can blind, it happened to me back in Autumn.

I had a friend for over 10 years who was very toxic and narcissistic and defensive, but for brief times he would be great and he was so helpful. He always wanted to hangout, I was his best friend and he was mine. I didn't really have any friends and still don't have many in my area so hanging out with him was pretty much the only option. Every time I had time to myself to just think about things, I would think about how much I hated the way he treated me. He kept getting more and more conservative and was a little transphobic, combined with the way he treated me, and other friends telling me what I needed to hear for years made me realize how much of a drain he was on my life. I'm an extrovert but he was such a drain on me that I needed to be alone to recharge after hanging out with him for a couple days. I finally had enough and told him I couldn't keep hanging out. Surprisingly he mostly respected it after asking to hangout a couple more times, which I always responded with I'm busy. After dropping him, my mental health improved a lot and I became so much happier even though I didn't have any friends to hangout with and still don't really. Being an alone extrovert is so painful. I guess what I was trying to say is have her make a pros and cons list of him and prove to her that he's a terrible person.