Similar thing happened to me OP.
My DH was a smoker, about 10yrs ago he said he 'gave up', after I pleaded with him to stop as it was expensive and dangerous for his health. I do believe he did give up for a while but at some point he started smoking again (I caught him a couple of times at our house), however it was hard to 'monitor' him as he worked away 4-5 days per week (including weekends and nights).
I didn't do anything about it and I pretended not to know he was smoking because
a) he was a non drinker so I stupidity thought that would cancel out the health affects of smoking.
b) He never smoked in front of DD or I, so at least we weren't exposed to it (a closet smoker)
c) And obviously it was his body, his choice etc and I knew he was a smoker when I married him.
To cut a long boring story short, DH died 4 years ago (hereditary condition so not caused by smoking but smoking certainly would not have helped) and on the night he died we found cigarettes in his trouser pockets, to say DD was disappointment is an understatement as she thought he had given up, she was so proud.
In hindsight, I wish I had done something and at least keep supporting him, not bury my head in the sand and let him smoke behind my back. But there was very little I could do, you can support him all you like but he needs to make the decision when he is ready to give up smoking, not you nagging at him to give up.
Because at about the same time, I was watching my brother (another smoker) try and give up smoking by various different methods, however he was not in the headspace to give up. So my brother is still a current smoker.