<human>: Can you give me the gist of the text in a nutshell?

Basically like the title says. They blatantly were all over each other while I was a few feet away the whole night. I know I don't trust my friend anymore and I've already decided that the girl is out of the picture too. However, her friend [21/f] is into me and invited me to spend the night last week, but I turned her down because I didn't want to start drama. 

Well now I feel like it would be sweet, sweet revenge to sleep with her friend and get back at her for sleeping with a guy in my close friend group. The girl I had a thing with and her friend are pretty close and hang out often. 

My question is: Should I sleep with her friend or should I just let the whole situation die? I feel like sleeping with her friend would be amazing on so many levels, but I've gotten mixed signals from my friends.
<bot>: Girl I've been talking to got with my friend in front of me, should I sleep with her close friend for revenge? (Obviously just a one night thing)