Supposed 'best friend' dating ex boyfriend!?

She obviously doesn't care about your friendship as much as you do. If she did she wouldn't have even considered getting with him. Or she would have asked you before. But would you have been okay with it either way? She should understand that your hurt by what she did and if it was the other way around she wouldn't have been okay with it either. Talk to her and tell her how you feel. If you don't want to lose her as a friend think about how she didn't care if she was going to lose you when she got with your ex.