<human>: Can you summarize the given text in a few sentences?

This guy, named Ed, used to date my best friend. We have a close group of friends and we all love her, she was our friend before she dated Ed. Anyways, Ed and Cindy had a MESSY break up, and all her friend got dragged into the middle of it. He did some terrible things to her. It was horrible and terrifying. Physical violence, verbal threats, vandalism... you name it, it happened. Total hell for her and all her friends involved.

So, this weekend we find out that Cindy went back to her ex one drunken night recently and had sex with him. After all the shit she put ALL of us through, she goes back and sleeps with him?! She kept it a secret and didn't plan on telling us, but his lunatic self though she loved him again, gets all crazy when she says it was only a hook up, threatens to tell all of us so 'we will hate her'. I think thats the only reason she told us because she figured it's better we know from her then him.

I told her to not worry about it. She apologized, and seemed upset and worried we were all going to disown her. I said to forget it and move on. But truth is.. I'm upset. She put our group of friends through hell for her. We stuck up for her, protected her, people even were physically hurt for her to save her from him. Then she goes and sleeps with him because she's lonely.

Do I/we have a right to feel upset?
<bot>: Best friend's ex is insane. Put her and our group of friends through hell (physically/emotionally). She runs back and sleeps with him 6 months later. Do we have a right to be upset?