Help on what to expect for lung cancer caregiver

My mother was diagnosed a year ago with adenocarcinoma stage IV. We thought last Christmas would be her last, but here we are again. She is 88 and was in good health. Her oncologist, without ever seeing her, said chemo or radiation were not options. He offered Keytruda. He also is not an M.D., but a D.O.. Not sure that matters. Anyway, mom would rather not ask questions about her future. She has no pain, just occasional shortness of breath. But she is not wanting to eat much. She keeps losing weight, nothing is appetizing to her. Is this from the cancer or maybe a side effect of the Keytruda?
Has anyone been through this that is willing to share their experience? I want to help her but not sure how. She gets a PET scan every 3 months. The cancer is slowly growing. Her labs are all good except she has what she calls a “lazy kidney” she was told. Due to media and her brother’s experience with chemo, she has always been against it. She large family has lived well into their 90s, and I feel if she had a different doctor, maybe chemo would have been beneficial to her but now she is against it and her doctor says she is not a candidate, so Keytruda is all she gets.
She is losing weight, has little energy.
How can I find out what to expect?