It is very risky to operate on an individual that is in congested heart failure mode. Almost always the heart is enlarged and the low cardiac output from heart failure severly compromises general health and fast recovery.
Unless it is a life or death situation (usually individuals with acute heart attack), it seems the best treatment would be medication therapy for any symptoms. The COURAGE study supports this advice, and the results of the study indicates that medication, stent implants and bypass does not extend longivity but merely treats the symptoms (emergency aside).
It is difficult to evaluate a condition for treatment without a health history, general health, etc. and your mother's doctor would be the best source for making a decision. I can understand your concern that the remaining years can be disabling and a serious reduction in the quality of life as well as pain, possible loss of memory, impaired rational thinking (not uncommon subsequent to being an heart, lung , etc. after being on a heart-lung machine).
The doctor should weigh the benefit vs. risk, and discuss the matter with family members. Hope this provides a perspective that can help with asking the right questions with the professional caregiver. Thanks for sharing your mother's experience, and if you have any further quesitons you are welcome to respond. Take care.