By "Let her die" you mean "Kill her", right?
You could just remove her from life support, then you wouldn't be doing any action to specifically kill her.
I think it would be much kinder to kill her using some sort of painless drugs than to let her die by removing life support or feeding tubes, whatever it is. I think we all need to be a bit more honest about our mortality - we're all going to be dead soon enough, and 'life after death' is a nonsensical and psychologically self-serving fairy tale. So lets just try to minimize pain and suffering.