The rule of friends and ex's

There's an unspoken rule between girls that states you never date a friend's ex-boyfriend. So what happens when a friend breaks this rule?

Over Easter weekend, I found out that one of my friends asked my ex-boyfriend to take her to her sorority formal. She goes to school in Virginia my ex-boyfriend goes to school in Pennsylvania. When my ex-boyfriend told me he was going to take her to her formal over the weekend, I was shocked. Mainly because, well, they barely know each other and she didn't even have the decency to tell me herself.

Should I be angry at her for not asking my permission? Should I be angry at my ex-boyfriend for taking her? Should I be worried that there's something going on between them that I don't know about? All of these questions and thoughts controlled my mind all weekend and I still don't know how to react to this situation.

Yes, I have another boyfriend. I've moved on. Still, it's the principle... isn't it?