How experienced a quilter are you?
You may need help to get a cathedral quilt done before Christmas if you have never quilted much before. Why not try something you know you can finish and later embellish it to make it more personal?
I've been working on some quilts (most are fairly simple) but because I've gotten stuck, I've missed some (self imposed) deadlines. Not knowing your grandmother's age or health, I think she would really enjoy working with you on a quilt that the two of you can do so it can get done.
Later after she passes, you can get it back with all of the fun memories of how she worked on it with you, making it even more special. Just a thought.