I can understand you being annoyed, he is a man whore, she is probably not in love, but for now on you need to be straight forward with her and have her realize who he really is and pursuing anything with him would be pointless. Seriously, it has been over a year.
No matter what you do, SHE will judge this year's birthday and how it panels out, but be there for her. Try doing something that keeps her mind occupied. Will you both be in school on her birthday?