If she really is your BEST friend then she would know you still have feelings for him, and so it was wrong of her.
If she likes him, she should have asked you how you felt about her making a move etc...
I would talk to her and ask if she's serious about him. Say you feel a little awkward and you dont want to see him hurt if she is not for real.
I KNOW it is incredibly hard in this situation, it happened to me too. I personally wanted to punch my friend, but i just hurt inside, quietly. You'll be fine one day, it just takes time to let go.
Good luck and hope all goes well.