(((hugs))) pinksoil!!
I haven't known anyone personally, but I have cared for those with emphysema (if that makes any sense). I'm a CNA - Certified Nursing Assistant. I have seen many go downhill and it is hard and painful.
They often have trouble breathing deeply. It makes them cough very productively, which scares the heck out of them! Understandably so though...it's not fun at all

Even with an oxygen tank, and scheduled lung medicines, their oxygen level is right around 89-91. (A normal person is at 90-100, closer to 100 though.)
It is very hard. My technique would depend upon her personality. Some people are straightforward, give it to her straight if that's her case. She's going to die very soon if she does not cooperate immediately. If your mom is nervous and standoffish, present the facts very slowly and gently. Assure her throughout that you love her very much and don't want her to suffer - that's your #1 priority. But you will be there for her throughout her struggle of giving up smoking after so many years. Pick a new thing that you two have always kind of expressed some interest in trying (or checking out) ~ a new thing for her to put her energy and thoughts into. Just a thought.
Very best wishes to you and your family!!
Shez