It depends. If he broke up with you because she was scheming to get him, then clearly you need a new best friend and it's a good thing he's out of your life too. They did you a favor. If he just liked her more after he got to know both of you, I don't see how it's your best friend's fault. Is she suppose to be alone and miserable because you are? Are you going to stop your best friend from finding someone who could be special just because that person is your ex?