By not being shallow. Why should it matter? My GF is flat chested; she's the best woman I've ever known. If a man chooses a woman just for the way she looks, their relationship will never work or last. However, if she is intent on changing her body shape then, by fully researching the subject and offering both sides of the argument, you must help her tread her path; walk with her and support her all the way.
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Boob job
Or look at completely differently: that it is not her breasts that are the problem, but her "hatred" of them. Or actually, it is probably not hatred - but rather she is sure she is disappointing you by having so small breasts. If she is not then you can fix it for her, by making her sure she is fine. You will both be much happier about that.