Yes, yes and double yes! Double mastectomy!
Give your Mother a hug from me! Best wishes.
I'm going to ignore that nothing is proven with this type of treatment and approach this from a commom sense standpoint. I definitely would say "YES" .... since your Mother is obviously in fairly good health and doing well a mastectomy is not really such a big deal these days. Bone mets is the easiest to treat and control and gives the best chance for a longer survival rate than mets to other organs. I would be as aggressive as I could be in the hopes that this might help prevent or at least delay mets to other parts of the body. I personally know several Stage IV survivors of many years who have a very good quality of life. It just seems logical that one wouldn't choose to leave the primary cancer site in place ..... at least that's my take on it. Kindest regards to you both.