how do she feel about him? do she still have feelings for him?
do you live with her?
you have to sit and talk to her and lay ground rules. your in a relationship with her and her child's father should only be in his child's life, not yours. i've been through something like this before, and my ex refuse to put his ex in her place. he was her friend and she was always in our business, but it was his fault for allowing her in our business.
you need to tell her how you feel, her ex boyfriend should not be a problem in your relationship. i blame her for allowing this. just like my ex couldve been a man and put a stop to it.
i wouldnt deal with this, or stay in this situation if she refuses to created a comfortable situation for you. i would rather two parents be separate then together arguing and miserable.