If she had already seen her grandmother's decline, she will probably not be too surprised by her eventual death. Considering that she lives hundreds of miles away, I would not tell her ahead of time, as it would just prolong the worry/desperation phase. When my sister passed away, I actually kept it from my children for months as one of them was fighting for his own survival. I was able to do that because she lived more than a thousand miles away and we only saw her once or twice a year and I attended the funeral without the children because my oldest's immune system was too compromised to allow air travel at the time. I think that when you make the travel plans for the funeral is sufficient time for her to get used to the idea. Grief has to run its course. You can be available to listen but she has to work through it herself.
