Should I be mad at my friends?

I have an ex, studing in the same college as me for the same major (she ditch me for another guy, and my friends know, in fact they told me). Lately Ive noticed that she makes plans to hang out with my friends, and they instead of evading they just agree to go. Should I be mad at my friends for not declining her invitations??? Or is this one of those things that will happen eventually??? What can I do?, please answer