Almost nobody sets out to undermine someone who is quitting. The lines that do damage are usually said warmly, by people who want it to go well. They land badly anyway, and it is worth knowing which ones and why.
Start with the worst, because it is also the most common: one will not hurt. It is usually offered as generosity, at a party or after a hard day, by someone who thinks they are letting the person off the hook. For a smoker in the first weeks it is the single most dangerous sentence available, because it is exactly what the part of their brain arguing for a cigarette has been saying all evening, and now it has arrived from outside with someone else's voice on it. That makes it feel like a second opinion rather than a craving. If you say nothing else on this list, do not say this one.
Next: you seem really irritable lately. Possibly true. Also useless, because irritability is a documented withdrawal symptom, they already know they are being short with people, and they are spending a lot of energy trying not to be. Naming it makes them feel watched and judged at a point where they are already low on patience. If their mood is genuinely hard to be around, the workable version is to say you know this stretch is rough and ask what would help, rather than reporting the symptom back to them.
Then: are you still doing that? Asked at three weeks, it tells them you assumed this was a phase. Quitting is difficult partly because they doubt themselves, and hearing that someone else doubts them too confirms the fear rather than challenging it. The same goes for any variation on we will see how long this lasts, even said as a joke.
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I could never do that sounds like admiration and functions as a description of how impossible it is. Said while they are trying to do it, it puts the task in the category of things people cannot do. They are not looking for awe. They are looking for it to seem ordinary and survivable.
You have done so well, are you sure you want to risk it by going out is well meant and quietly insulting. It treats them as fragile and it treats normal life as off limits. Quitting has to survive contact with parties, pubs and stressful weeks eventually, and the person who is going to manage that does not need to be wrapped in cotton wool at week two.
My uncle smoked until he was ninety. So did somebody's uncle. It is offered as reassurance and received as an argument against the whole project. Similarly, my friend quit and gained three stone lands as a warning, whatever the intention.
And the hardest one: if they slip and smoke a cigarette, do not say I knew it or well, that is that then. A single cigarette after two weeks is a slip, not a return to smoking, and what happens in the next few hours decides which it becomes. The response that helps is the boring one — that was one cigarette, it does not undo two weeks, what do you want to do now. People are far more likely to carry on quitting if the slip is treated as an event rather than a verdict.
What actually works is less interesting than you would hope. Ask how it is going and then listen to the answer without adding advice. Notice the milestone without making a speech about it. Offer a specific alternative rather than a general encouragement, which means suggesting a walk rather than saying stay strong. Take the change of habit seriously enough to adapt to it — meeting somewhere without a smoking area for the first month costs you almost nothing and removes a whole afternoon of difficulty for them.
And say the thing people rarely say out loud, which is that you know it is genuinely hard and you are not going to make it into a test they might fail. Most of the pressure a quitting smoker feels from the people around them is not disapproval. It is the sense of being observed. Take that away and you have already helped more than any sentence on this list could.