[lily.]

Personally, if my best friend went with my ex, I'd disown her. It's like the unwritten, unspoken rules. You don't go with someone your best mate has been with.

So you are right to be annoyed at your friend, definitely.

As for your ex, I don't think she's worthy of your feelings to be honest, cuz she clearly doesn't consider yours when deciding who to shag.

Just my opinion. Sorry if it sounds harsh.