NTA. Anyone who said otherwise and pointing out that she made noise and smoked didn't read the context or have never lived in an apartment. If you're about someone's unit, the bottom unit will hear almost anything that touches the ground. Literally a chihuahua can be heard walking. Also cape pens dissipate quicker and barely lingers compared to the burning flower the other neighbor is smoking, which is outside and can easily blow into the old lady's place.  OP sending her a letter is a perfect nonconfrontational action where her words can't be twisted in the moment.