I'm so sad to hear that, but I'm sorry - the only thing that you can do is be a good granddaughter, but if you were in the same country as me, I would give you and your Nan a massive squeeze. xxx
Just love her. If she feels the need to talk about it, let her. When my dad got cancer, he needed to talk about it, and no one wanted to listen. I was the only one who let him say how sad it was making him. Everyone else felt they needed to be cheerful for him. That wasn't what he needed. He needed someone to listen. Just be there for her.
Spend time with her and learn her stories and history.
You'll be making her feel loved and cared for and learning family history to pass on to your children one day.
Just hang out with her and put a smile on her face.