Make her a list of reasons why she should quit.
She probably knows them all, but tape it to her mirror, or the fridge, somewhere she will see it every day.
Include the fact that the money she spends on cigarettes could be saved for other things.
Include the fact that you love her.
Include that you know quitting will be hard, but that she's strong, and can do it.
Include all the things you want her around for - graduations, weddings, grandchildren, etc.
Include that you and she will have better skin, and better respiratory function.
Include the fact that she will be able to taste and smell thing better than she has in years once she has quit.
Include things that she, as a nurse knows, but may forget - like it's not always lung cancer, but pancreatic cancer that doesn't have the warning cough, and is silently deadly.

Repeat several times that you love her, and will help and promise to be patient with her during the quitting process ...