Karen,
at the risk of freaking out other survivors here the bottom line is that if it was a late staged cancer (like mine was), the 5 year survival rate is 38%. The immune system is pretty beat up because of treatment as it is, plus she had the low sodium thing going on which is a fairly serious complication of treatment. I am sure that the statistics include persons who don't necessarily die from the cancer itself but rather complications from it. There are many infections to be found in hospitals unfortunately, in spite of the best efforts of aggressive infection control. There may never be an answer. Even thrush can kill you if it gets out of hand and goes systemic. I know that several of us have had to be hsopitalized with IV antibiotics from otherwise rather mundane infections.

Like your Dad said - this won't bring her back and you'll be living the pain for years - but with a root of bitterness. You have to ask yourself also how would she like to be remembered.