Her height is a consideration too. If she is a tiny lady either choose a shorter length style such as Rosemary with its exquisite hem or a nightdress with a plain hemline as it can then be shortened if necessary without spoiling the look of the garment. If she is very tall opt for a longer length style like Nadine that will still look right if it is shorter than it is intended to be.
What is Mary's bodyshape? Be sure to choose a cotton nightdress that will give her room to be comfortable in bed as most of the pure cotton styles don't stretch but in general terms always choose one to fit her bust rather than her hips as most of my nightdresses are intended to be full around the hips.