My BF dumped me for my BFF!
This is a messy situation you have on your hands, chica.
Before you go and dump two people out of your life for good I think you should
have a private talk with your best friend. Ask her why she would date your ex
right after he broke up with you. There is a chance that she might have thought
it would not hurt your feelings. You need to express to her that your feelings
are indeed hurt. If she is sorry and expresses her concerns for what she has
done then I think it is OK to keep her as a best friend.