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Having been way too close to this sort of situation, it is my bet that the boyfriend forced this ultimatum as a way of cutting his girlfriend off from a main source of support. He probably told the girlfriend that "I'm not going to marry you with that bitch friend of yours as a maid of honor unless you can get her to change her tune". Abusive men (and he sounds emotionally and possibly in the future physically abusive) thrive on the power they get from cutting the woman off from the rest of the world, so as to impose control more effectively. With this ultimatum, he wins either way: either the friend has to back off resisting the marriage or she's out of the picture as a friend and not a threat to him.