<human>: Can you condense the text into a few key points?

I'm using a throwaway because my ex boyfriend uses reddit and knows my username.

The title says it all, really. I was head-over-heels in love with my ex, we dated for a year, and he broke up with me about 6 months ago. It was not mutual at all, and he was very cruel to me in the process. It's been a very tough time for me, and my ex's true colors showed throughout the break up and post break up. I found out a lot about how much he lied to me throughout our entire relationship, and he is just not a great guy.

My best friend was by my side through the whole thing. She knows every single terrible thing that he has done to me, and she knows better than anyone how devastating the situation was for me. 

My ex finally told me last week (we hardly speak) that my best friend has started pursuing him. She shows up at his house constantly and they spend a TON of time together. He doesn't have many other friends. It should be noted that my best friend is a huge flirt. 

I'm not sure how to proceed here. I texted my friend yesterday and we made plans to talk this weekend. I don't think that she knows what it's about. She has barely talked to me at all the past few weeks (this is a huge change from what our relationship was previously like). If anything happened between the two, it would honestly kill me. The thought of him with anyone else makes me sick to my stomach, and the thought of him with her... is just too much to bare. I don't know what to say to her because she's one of those girls that will just continually say I'm being silly and make me feel really stupid about it.
<bot>: my best friend has been spending a lot of time with my ex boyfriend and they might have feelings for each other. I am confronting her this weekend. What should I say?