Make a plan with friends beforehand. Decide what you each will accept, and stick to it. Don't leave in conflict. Leave a chance to still do things together instead of making things tense. Totally change the focus of the conversation- change the topic, ask an unrelated question, give a compliment.
Studies show that friends or family can have a real impact on a smoker's chance of quitting.
You cannot MAKE your child stop smoking. What you can do is explain your concern, show you care and say you will help them if they want to try and quit. One of the best approaches is to ask your child what you could do that would be most helpful. Sometimes it is hard for them to know the answer. Listen well to what they say.