He chooses who he chooses. Express your dislike for her in the most non-offensive way possible, but don't bug him about it and how you want them to break up, because it is really none of your buisness.
You could start a bit of trouble too, a friend of mine decided to bag my GF out once, so I chucked my ciggerette in his face and stared him down, despite being twice my size he got the point and never even made a joke about me or anyone I care for ever again, point being, it is offensive if you don't say it properly.