friends vs ex

a few years ago i was in a pretty bad relationship. there were definitely abusive parts of it. i did get out of the relationship, but it took literal months and a lot of energy. when i started trying to put up boundaries in the beginning, it was like all the emotional abuse heightened. it was a really hard time, but i made it through with the support of my friends. since then, though, my best friend has been spending time with my ex. it’s platonic, but it still hurts that she would do that after knowing what happened in the relationship. this friend was the first one to sit me down and say she was seeing signs of abuse in my relationship. now, she hangs out with my ex, she does tell me about it, but only several days after it’s happened. she was friends with my ex, and says she doesn’t want to hurt her feelings by cutting ties. what should i do?