I would advise the mother to stop breast feeding from the breast where they think the cancer is. Not because of any danger to the baby, I don't think that there is any danger unless the breast is infected, but rather because of theoretical danger to the mother. I don't think that this has ever been proven, but I've always thought that if you shake or manipulate a tumor too much and break it up, it makes it harder to cleanly excise and you increase the chance of metastasis. That would make sense to me anyways.