Teen relationships

What do you do when you break up with your ex but still have feelings for him and your best friend goes and talks to him but he hurt you really really badly. Do you drop the best friend or do you get over it? Personally I told my best friend how much my ex hurt me and how we weren't on good terms and she still went and talked to him. She never told me. My ex accidently sent me a photo of him talking to her (probably on purpose) and when i confronted her she was going on about how it wasn't right and she knows it but she is still going to talk to him. What do I do?