I can kind of relate to the situation that you described because my 80 year old mother came off of 6 months of intensive chemotherapy for Hodgkin's Disease last September.
A follow up CT scan and Pet scan, two months following that, showed that she had an enlarged lymph node in her right groin that had apparently not responded to the chemotherapy. A biopsy of that swollen lymph node came back positive for a low grade form of Non-Hodgkin's Lymphoma. So, now she's battling another type of Cancer.
Anyway, where I'm going with this is that... whenever these serious/ life threatening and complicated medical matters occur, some people seek a 2nd opinion... and, I think that there is immeasurable value in that. My mom got the opinion of 4 different oncologists as to how to approach the newly discovered Lymphoma. And, all 4 were equally divided as to whether or not my mom would benefit from radiation treatment... 2 said, yes; 2 said, no.
Her prognosis without radiation...10 years. So, she opted to not undergo radiation. She's content with her difficult decision to decline further treatment and, like your mom, she figures that "... when you gotta go; you gotta go." But, she had to "pull" that prognosis out of the doctors in order to be able to make that informed decision and plan for whatever is left of her life.