<human>: Me and my girlfriend dated for about 3 years. We had a mutual break up last Nov, because we both wanted different things. After the break up we still kept and touch and soon became FWB (Friends with benefits). Basically for us this was a long break before we got back together. 

Things seemed the same as they were when we're dating. She made it open really open and we could fool around with others, but in the end we came back to each other. (Note* only she was the only one playing around).  I wanted to stay loyal towards her.

When we would always hang out with our friends, she'd constantly be flirting with my best friend. I know I have no right to interfere, but it was killing me inside. My best friend told me he would never go after anyone's EX. But seeing how he was acting towards her I had second thoughts. Throughout the months he's been acting more cold towards me and extremely close to her. I tried talking to her don't do this, but her reply was you should've told me before I 
started it's too late now. 

She told me she didn't like him at all and just wanted to play with him. Constantly she'd provoke him as far as she could, until he did something. I kept telling her if you keep doing that he'll like you... In her mind that made it even more fun for her. I was sitting back watching this happen in front of my eyes.
My best friend was getting more intimate with her as we hang out.

Now she tells me she wants to have FWB with him, but only if he doesn't like her. I constantly tell her he does by the way he's been acting, and how she is leading him on. But she won't believe me and is going to ask him soon.  If this did happen and in a year and my EX get back together, will things be the same...

NOTE*  My best friend talked to me a few months ago saying don't worry there is nothing going on between me and her.

Can you summarize the given text in a few sentences?
<bot>: Ex acting very close to my best friend, which supposedly won't do anything. (PS me and ex are still seeing each other)