Tricky question... If you accuse her of flirting with your boyfriend she will probably deny it and call you paranoid, jealous or something like that.
Tell (don't accuse) your boyfriend that you are under the impression that your friend enjoys flirting with him and listen to what he has to say about it. If he confirms that she is indeed flirting with him you can tell her that he told you she is flirting with him. She will get the message and will not be able to make you feel that you are the one being paranoid or jealous. In addition, you can tell her that if it's true (that she is flirting with him) that she is hurting your feelings and that it is not a way for a friend to behave.
If your boyfriend denies that she is flirting with him you may either be reading too much into her behavior or have reason to believe that your boyfriend likes her as well and would rather lie to you about his feelings than tell you the truth about how he feels.