Your mother is a very strong and courageous woman. She has fought this disease with all her might and I'm sure much encouragement and support from you. There comes a time when we have to accept the fact that no one is getting out of here alive. Something will do all of us in at some time. The final decision on treatment must be hers with the advice of her doctor. Chemo severely impairs the immune system and at her advanced age and debility from the cancer and chemo, it's hard for her body to recover. She could try the radiation, if the doctor thinks that would be of help. Have they done PET scans or other imaging? Those will show the extent of metastasis.
I hope I haven't been upsetting or unduly direct, but we must be mindful of the suffering and quantity is not necessarily better than quality of life. Take care