I would let her do what she wants to do. She is 90 years old and has earned the right to do as she pleases. You may ask her to quit, but she has earned the Right to do as she pleases. IMHO
At 90 years old I think she has earned the right to decide for herself about smoking, or anything else that might crop in her life.
If I were 90 years old, I would eat, drink and smoke whatever I wanted, whenever I wanted, and my attitude would suck towards anyone who tried to stop me.
Giving up smoking, at 90 years old, will most likely serve to make her unhappy, discontent, and just plain mean. Unless, of course, she has higher than average logevity in her family, at 100+years.
My advice to you....let her make her own decision about the smoking thing.....afterall, she's 90.
My advice would be to tell my grandmother to quite smoking for the benefit of the rest of the family who care to see here around for another 90 years. I dont want to say plead with her, but at least tell her that you love her, want to see her around, and are dissapointed that she does not care enough about the rest of the family to stop. Of course, if you smoke, you will need to quit too.