Your message mentioned the surgeon was unable to remove the entire tumor. If that's the case, the chemotherapy is palliative care only. From what I understand, the main purposes of using it are to reduce size of tumor and therefore help control pain and other issues associated with PC. The side effects of the treatments have to weighed against the discomfort and pain she may otherwise feel. Also sometimes it does give the PC patient extra time.
I am a PC survivor. My husband suffered terribly while I was going through treatment. Sometimes I think the ones watching suffer most because they feel helpless. But the experience I want to share is this: My Father-in-law was diagnosed with inoperable lung cancer at age 75. The doctors told him he had 6 weeks to 3 months maximum to live. He pursued every possible treatment, including an experimental therapy at Emory Hospital in Atlanta. He was alive one year after diagnosis and still living on his own. The treatment was never represented as a cure. My brother-in-law asked my husband to go with their father for one of his check-ups at Emory. When they got there, my brother-in-law asked to speak with the doctor without Dad in the room. He insisted that the Doctor tell my beloved Father-in-law that there was no cure for his condition, that there was nothing else they could do for him, that he was dying. It's true there was no cure, but they would have continued the experimental treatment if Dad wanted it and continued to tolerate it. The doctor did as asked, and my husband told me he saw a change in his Father during the conversation. That was on Friday. I spent Sunday afternoon with Dad, and he was very quiet, telling me he had a lot to think about. My Father-in-law passed away the following Friday, one week after the doctor told him there was nothing that would stop the cancer. My brother-in-law did what he thought was right, and my husband did not disagree with it They both suffered along with Dad watching him struggle to go through the treatments. I've always wondered if the doctor had just told Dad he was doing OK, or as well as could be expected, if he would have lived longer. I know Dad wanted to live every day he could because he always held hope of a breakthrough. The real question I will always have is if insisting the doctor tell Dad the "truth" was for Dad's benefit, or the benefit of those that loved him so dearly, who wanted to see the suffering end.
Sorry for the long post. Knowing what to do in these situations is difficult. I hope your mother in law finds comfort on her journey, and that your family is able to find peace whatever the decisions taken.