Have you caught your best friend in a rebound relationship with your ex? This happens sometimes and it is probably making you really angry.
Who is to blame for this, did your ex hook up with her? Or has she been wanting this to happen even while you and your ex were together? I think you would not find a true friend in a relationship situation like this.
A true friend would be on your side helping you to cope with the breakup.
If you have the desire to get your ex back, you do have a chance.
First off do not say anything about your ex being with your supposed best friend.
If you talk you do not need to say anything that would create tension or start a fight.
Just play it cool and wait it out.
There is a really good chance this couple will not last a rebound relationship.
If your ex initiated it, it was because they felt they needed to fill a void in their life.
If your friend started it, they were taking advantage of the situation.
When the time comes that you and your ex can discuss your situation, do not bring up any thing from the past that would start a fight.
If you were at fault for the breakup apologize for your actions.
If your partner was at fault apologize anyway for something you did, this will make you look like you care about the two of you.
There is no guarantee that you will get back together, but if you do not try you would never know.
You have nothing to loose because you are already broken up.
There are lots of books out there that can help you with some proven methods for getting your ex back, check them out.