Is it normal that i will not forgive my best friend for this?

So I met a guy, I had been hurt (badly) recently but decided he was worth the pain. Really fell for him, he often said the feeling was mutual. We spent time together, had sex a bunch etc. Then one day, he stopped speaking to me, out of nowhere. People had warned me he was a mean person but I thought I knew him. I was seriously upset. I became friends with a girl during this time and she knew about the heartbreak. Overtime, we became best friends.

She would often talk to this guy (that she had met through me) on facebook. One night, I got drunk and slept with him. He showed me multiple messages of her propositioning him, and getting jealous as she knew he was with me. He told me they had spent some time together, and that she had promised him that she had asked me and I had told her I was ok with it. She had been lying to me and seeing him secretly.

I was angry but decided to bring it up with her, forgive her and let it go. She tried to tell me that she had only invited him over to stop me from going back to him and getting hurt again. Claimed she was only trying to distract him. Everything was fine until she continued speaking to him all the time! So I asked her nicely to stop, she agreed.

Recently, she had been writing on his wall more than usual. On friday night, she ditched me last minute. She later told me she was with him. I got angry at her and we texted each other during the next day. I explained why I was upset and she told me that she would stop seeing him, out of respect for me. I was angry but knew all would be forgiven.

Later that night, I found out she had invited him over, and that he was at her house. I was seriously angry and asked her never to speak to me again! Most people are on my side, but some are saying I need to hear her out and let her explain. Do I really owe her that?