Friends and ex boyfriends

So recently I found out that my friend had hooked up with my ex, she told me she was just drunk and other things happened that night but now she wants to take things further with him and he's admitted he likes her. 
When I found out, it felt like I had been stabbed in the heart. 
I am in a wonderful relationship, and it's not that I am jealous or have feelings for my ex boyfriend. But I am 100% against friends dating ex boyfriends, I've just always been like that? It really did hurt me and I felt betrayed, and now I'm starting to feel like an idiot.
What makes me feel even worse is that even during our relationship, I told my friend I felt threatened by her and I was worried they would have more than a friendship and now it's actually happening. I've explained everything to my boyfriend and he also feels like she shouldn't be doing this and I have a right to feel upset. But I'm really not sure what to do, my friends are my life but I hold the decision wether they are allowed to date or not and I shouldn't have to, because I feel like my friend should know better. So I'm not sure what my question really is, but just any advice, at all?