Les,
First of all, your mother's age should be of less concern than her present state of health. The recommended surgery may not be as disfiguring as you think and that is relatively unimportant when weighed against the possible effects of chemo given her history. I had most of my left mandible removed and opted not to have reconstruction at that time. I have posed for an advanced portrait painting class with people who are extreemly observant and those who did not know I had the surgery did not realize it.
It seems that your mother is under the care of a competent team of doctors. Are you getting information first hand from them, from her or from another family member? What is her opinion about the options? How does she feel about your input? Did the doctors recommend the reconstructive surgery for functional reasons? It would seem that "doing nothing" is the least option. Have you spoken with a clergy person and/or a social worker?
Please consider the advice that you will receive from others posting here. Our experiences are all different but we are facing the same basic issues.
Malka