I want my ex to stop talking to her friend.
Why don't you want your ex talking to this friend of hers?
The friend is the reason why she won't be with me.
I understand you're probably still hurt about her not being with you any longer.  That being said, you're not in a relationship with her any longer and who she speaks to should be none of your business!  You need to move on.
I'm not going to move on.
Being so hung up on your ex is not healthy. If they don't want to be with you, then you need to accept that and try to move on with your life. Finding someone new to date will help you realize that there are other people out there.