Im unfamiliar with the nuances of the situation but id say you should sit your partner down and talk to them about why exactly this situation bothers you, theres a possibility that nothing is amiss, they are just friends, and that issuing an ultimatum of "its her or me." will only have negative consequences.
then again dont make an effort to be unaware if something IS going on. this doesnt mean launch surveillance and keystroke devices and whatnot to track his activities, but it does mean keep an eye out.
lastly dont be paranoid. if youre looking for something wrong you will find it whether its there or not. im 100% faithful to my girlfriend and she still digs around my facebook messages and emails for anything that might be incriminating and it drives me insane. hell, odds are really good that she'll find this comment and say "WHY WOULD YOU TELL PEOPLE THAT I SNOOP ON YOUR FACEBOOK!" or something like that... anyway i digress.... love you emily.
talk to him, trust him and consider what he has to say, dont be paranoid, but be cautious.
If i wasa girl i know exactly how id deal with it, but my way would hurt lol and its by no means the correct one either. so my advice would be to trust your boyfriend, and ignore her till she puts her foot in it, then put your foot up her ass.
I personaly see a few logical answears,
A.) Tell him how you feel, and see if you can possibly come to a compramise. Many people are great friends with their exs becouse they just are, an ex knows your ins and outs, so he probly uses her for talking about some issues that he dosnt want you knowing hes having, he probly uses her has a sound board, meaning he bouncies ideas off her and see what she thinks before talking to you becosue he wants you to think he always has great and thoughtful ideas.
B) get over it, I dont think hes going to ditch her just beocuse you dont like her, and you havnt met her, ok, so she dosnt like you, but shes just jealous, meet her, hang out with her, see what kind of person she really is.
C.) leave him if it bothers you so much, then you wont have to deal with it.
and D.) Kill her, make it look like an accedent and be your bfs shoulder to cry on.........