About 12-13 years ago, my wife's grandmother had a series of strokes. She survived, but ended up in a Long-Term Care Facility(what I would call a nursing home), where she has been on a long, slow slide. Physically she has been fine, but mentally, there has been less and less of HER there, to the point where she hasn't recognized my wife the last two visits. Rather(corrected by my wife for accuracy) she has recognized my wife as one of her grandkids...just not which one.
The last two weeks, things have taken a turn for the worse. She stopped eating on her own, and, basically, she is dying. My mother-in-law, and her siblings have decided to let things happen naturally at this point, a decision both my wife(and I, though my opinion is unimportant) support.
Agreeing that it was the right decision two weeks ago is a different kettle of fish from living with that decision now that she has slipped into a coma, and her time is being counted in days(maybe even hours). The last two days, my wife has been quite weepy...which I supposed is totally legitimate.
All of my grandparents were dead before I was about 8-years old, and I have not lost anyone that close as an adult...so I can't really imagine what my wife is going through. I know the last time she visited her Nana, she told me she had made peace with the fact that the woman in the bed was no longer the Nana she grew up with...but that isn't the same as saying good-bye.
For now...all I can do is be there for her. Give her some hugs when she is feeling bad, and when it does finally happen, make sure I don't do anything to make her pain worse.