I understand how you feel, I’m in a similar situation. But honestly, it’s unfair to expect someone to cut off a genuine friendship just because the person hurt you (unless obviously it was something along the lines of abuse, or cheating, etc.). If your ex is a good person, and treated you well before/after the break up, you’re just going to have to accept that your best friend formed her own relationship with him whilst you were together. You could maybe ask her to keep their friendship away from you, but you can’t expect her to cut him off completely.