I am just learning metastases as my father has it in his bones from prostate. From his treatment report, there seems to be 3 sections of treatment. One is the obvious one of the chemo which blasts the cancers and shrinks them. As you know its got some pretty nasty side affects one of which it messes with the bones. So his second section of treatment is Denosumab which stops the development of cells that break down bone. His third area of treatment is Zoladex injection which is in the similar family of inhibitors like Tamoxifen and Femara.
Each of the sections all help subdue the cancer progression but really a 4th part is that break from chemo like your mum is having. My dad had over a month off his and gained back some weight and really is looking good now. You wouldn't think he has cancer atm. :)