(Update) My best friend (25/f) thinks I (25/F) ruined a relationship and now she won't talk to me.

Original post.
I didn't think I would need to update especially after hearing nothing from John's girlfriend after the first couple of days but, here I am.
First off, thanks for the comments and PMs. I know I did the right thing and I was confused by Anna's reaction. But the support that poured in helped so much. Thank you.
Now, Anna still wasn't talking to me when my trip home came to an end so I just drove back to my city and went on about my life. She ended up calling me middle of last week and said she wanted to talk about what happened. She still believed that what I did was ****ed up and she didn't approve. I asked her if she would ever tell me if she found out a future boyfriend of mine was cheating on me and she said "Of course. But that's because I know you." I told her I didn't see the friendship/stranger status as an excuse to let a guy be ****ty to his girlfriend. After a while of back and forth with the same arguments I told her that we can agree to disagree and leave it at that. She said okay and we hung up.
After we hung up I decided to check my Facebook and low and behold there was a message from John's girlfriend in my inbox from two days before. I was kind of nervous to read it but it just said "Are you in ________ right now?" I replied that I wasn't and an hour later I got another message asking if I could call her and her number. I was super nervous because I had no idea what I was going to be walking into but I figured if it got too bad I could hang up and block her number.
I called her and it was a very awkward greeting and then she asked me to tell her everything that happened. I obliged and told her exactly what happened. I could tell she was crying on the other end which broke my heart and I started apologizing but she told me not to. She was grateful that I told her. Apparently John and her have been together for close to 4 years and he cheated on her a lot during the first year of being together until she found out. He promised he'd never do it again and she took him back. She said she was always really paranoid he would cheat again but he never showed signs or anything out of the ordinary.
I apologized for being the spotlight on what an ass he was. We kind of got to chatting for a while after that and after a while she switched from the depression to the anger stage and I let her vent. At the end of it all she said she was finally going to leave him, which I agreed with, and then said that if I'm ever back in town that she'd like to buy me a coffee. Which I thought was a nice gesture and I might take her up on the offer.
After the phone call I was really relieved and happy about how things turned out. I went to go text Anna to tell her but then I remembered that John's girlfriend said they were dating for 4 years. So I texted Anna and asked her when she dated John. It was two years ago. She must have known that he was dating another girl and that's why she was so ****ing ******. So a lot of you guessed right that she was his mistress for a while. I'm not going to tell John's girlfriend since she's done with him (hopefully) and doesn't need anymore heart ache. But I'm definitely going to cut Anna out of my life. And when she asks why I'll just tell her that I know about her and John.
All in all, it turned out okay for pretty much everyone involved except for John. Which is good because **** that dude.

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tl;dr: Lost a best friend but gained a possible new friend in John's ex.


Source.