Watching your best friend get her heart broken is one of the hardest things in the entire world.
You can't imagine why anyone would ever break up with her; I mean she's smart and funny and loving and a generally beautiful human being! What idiot would give that up?
This past year, I saw my best friend suffer that kind of heartbreak, and at first I was incredibly tempted to drive to his house and punch him in the face. Watching her go through her break up and trying to help her in any way that I could, I've learned a lot about controlling my hatred for idiot boy and focusing on being there for my friend.
First, be sure that she has ice cream. There are few heartbreaks that ice cream can't fix. Eat some junk food and cry together. Let her know that she can always share what she's thinking with you. Let her know that it's always okay to not be okay.
Don't start bashing him to her. As angry as you might be with her ex, bashing them isn't always the best way to make her feel better. Feel free to participate in an angry rant session with her if she initiates it, but let her feel how she's feeling in the moment. If she isn't angry, don't try to force her to be.
Take her out for a fun night. If she wants to take her mind off things, get the whole squad together for a night out. Get dressed up and go to dinner and let her know that your entire circle is there for her.
As hard as it can be to see your best friend upset, it's important to be there for her. Remind her how important she is and be as supportive as you possibly can. You know that she'd do the same thing for you.