You need to ask the doctors these questions but chemotherapy takes a MASSIVE toll on the body. I don't know how much you understand of what it does but essentially it targets any fast growing cells, i.e cancer cells. However chemo is indiscriminate, hair cells grow quickly, that is why chemo patients lose their hair. Chemo also destroys your white blood cells - those that fight infection so your risk of infection is greatly increased and blood platelets are also affected, preventing blood from clotting properly. If your mum is already ready weakened they cannot give her chemo because her body will not be able to withstand it. Even if she did still manage to have it, if she got an infection, in a body weakened by cancer, weakened by complications and weakened by chemotherapy it would be a total disaster.
I'm afraid I think you have to accept the doctors advice in this instance, concentrate on getting her stable and well enough to endure chemotherapy. It is not an easy thing, and in my opinion an appauling way to treat cancer. The idea is to kill the cancer before the chemo kills you. I'm sorry I could not give you better news but chemotherapy is very difficult, and you MUST not be afraid to ask her doctors these questions, you have every right to know.
Very best wishes to both of you and your family x