Ex and friend

So my boyfriend and I broke up about 3 months ago. We dated for a year and 8 months. During the last 6 ish months of our relationship, he was obviously crushing on one of my good friends. I was insecure about it and he knew it because I told him. Now that we have broken up he's always with her and have gotten really close with her. I don't know how she feels about him but she does a lot for him now too. The only reason I'm upset is because he knew I was insecure about her and if they start dating then my insecurities would be true. I've talked to her about it but it was about 2 months ago and things have grown since then.