Im so Confused

Say no if you don't like him and don't say yes just to spite your friend. Do not lower yourself to her level. Be the better woman and move on. Forgive and forget about it.

Good luck and best wishes. 

Hi,
Tiptopmanager gave you (as usual) an excellent answer.  I would like to add:  Once you broke with your ex he is "free"..... If your best friend was a bit more sensetive / considerative she wouldn't date him....  In fact she did it and that means that she is NOT sensetive / considerative..... (and you could avoid it by telling her the truth [that it hurts you])..... please remember that from her point of view she did nothing wrong.....  You are different, you are much more sensetive / considerative and with higher E.Q. so don't go down to her level..... Besides, as far as I can guess you won't be able to go out with someone you don't like.....   Yes, you can get back to her but always remember that she is not a sensetive / considerative person (and that has it's consequenses).....  or, get yourself better friends...... friends of your style / values (etc).  
Best regards,

You have what is called a knee jerk reaction. That means you feel totally justified to get revenge because you were so hurt by your best friend. But revenge is destructive and will prevent you from moving on with your life. So don't waste your time. Find a best friend who you can trust and if your boy friend asks you out you can politely decline. At your age you have a whole life ahead of you. Build it by making friends with good people and avoid those who aren't. This way the deep hurt and anger you feel today will soon slip quietly into the past.

No revenge does not solve anything , if he hurt you in the past its best that you stay clear of him.

I have to agree with Brosen. Instead of seeking out revenge against this alleged friend of yours, I say be the bigger person and let it go. Move on with you life and find new and better friends (that won't date your ex-boyfriends).