I'm slowly getting there with quitting and it is bloody hard.
I'd say talk to them when they do have a smoke, just mention that they're trying to give up and tell them they're doing well. Of course be careful how you word it so it doesn't come over like you're preaching.
For me personally I hate it when people preach about it, especially people who have never smoked.
Our lorry driver at work, admittedly he his a cantankerous old git, he's never even touched tobacco let alone smoked it and he is always preaching to the smokers about how it's bad for you and it really annoys most of us, of course we know he is right but it's the way he says it. What makes it worse is how he goes on about it unhealthy and in reality he is the most unhealthy person there, he's always coming down with colds and chest infections and needing to go the doctors and I know I shall talk one up now, I haven't had a cold in about 3 years and the only time I go around the vets is when I have diabetic checkups.
Anyway I personally find that when people preach it kind of puts me off the whole quitting, stupid I know but I think it's just human time lord nature.