Be clear that you dont approve of them smoking or using e-cigarettes, and that you expect them to live tobacco-free
However, there are a number of instances where Chazal applied the pasuk from Tehillim , Shomer Pesaim Hashem ( Hashem guards the simple) as a principle to permit an activity, despite a possible risk to ones life, when society views the level of risk as negligible (see Yevamot 72a, Shabbos 129b, Avodah Zarah 30b, and Niddah 31a)
And understanding how it is affected by smoking, and what you can do to help is just as vital
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"What she's doing is actually working," he said
You can also do this right into a proper waste management receptacle