That's just horrible. And the "cure" may be - well, not worse than the disease, but...
Let me guess: Chemo, radiation and (not or) bone-marrow transplant?
How old and - more importantly - how agile is she?
When he was 80, my FiL had an aggressive type of blood cancer, first falsely diagnosed as a skin matter. When finally diagnosed correctly, it was too late already - but they tried chemo and radiation therapy nonetheless. Despite the fact that the cancer felt like being skinned alive, after a few sessions he preferred to return home to his wife and spend the rest of his life in dignity. We lived in the same city and took care of my MiL after his death (she stayed in their own apartment until her death many years later).
It's really, really hard - but ask your Mum what she really wants for herself and assure her that you stand by her whatever she decides (even if it's heartbreaking for you).