So, this whole situation feels like something out of a bad soap opera, but I need some outside perspective because now my friend group is split on whether I’m overreacting.

A year ago, I broke up with my ex (28M) after finding out he had been cheating on me for months—multiple girls, constant lies, the whole nightmare. I was devastated, but my best friend, “Emily” (26F), was there for me through all of it. She was furious at him, called him a manipulative asshole, and swore up and down that I deserved better. She was the one wiping my tears, telling me I’d heal, and promising she’d never forgive him either.
Fast forward to last week.
Emily started acting weird—canceling plans, being dry over text, and just overall distant. I figured maybe she was just busy, but then a mutual friend casually mentioned that she had seen Emily out at a bar with my ex. Just the two of them.
I immediately texted Emily and asked if she had seen him. She took hours to respond and finally said, “Yeah, I ran into him. It was totally random, we just caught up for a sec.”
Something in my gut told me she was lying.
And I was right.
A few days later, another friend of ours let it slip that Emily had actually been seeing my ex for weeks—and had even slept with him.
I confronted her immediately. At first, she tried to deny it, but when I laid out everything I knew, she finally admitted it. And then she hit me with:
“I was going to tell you, but I knew you’d be upset.”
Gee, wonder why.
Then, as if that wasn’t enough, she started defending him. Saying he’s “changed,” that I “only know who he was,” and that “sometimes, love just happens.”
I told her she made her choice, and now she can live with it—I want nothing to do with either of them. I walked out and haven’t spoken to her since.
Now, here’s where I might be the asshole.

A few of our mutual friends think I’m being too harsh. They say that while it was a shitty thing for her to do, “she didn’t owe me loyalty” when it came to who she dates, and that I should be happy she at least “didn’t sneak around forever.” Some have even said I should be “more mature” about it and just accept that people move on.
I don’t think I owe her forgiveness, but now I’m wondering – AITA for cutting her off completely?