Contrary to what your kid's friend says, e-cigarettes are not safe. You are still inhaling nicotine, meaning that you are still susceptible to many of the adverse effects of regular cigarettes, i.e., addiction, cardiovascular problems, increased risk of cancer, increased risk of birth defects if you use them while pregnant, and so on. In fact, liquid nicotine, the active component of e-cigarettes, has been historically used as poison in assassinations: if you coat the blade of a small knife in liquid nicotine, even a small cut can be lethal, as a mere 60mg (about .002 ounces, about the amount you can fit in a couple of drops) are enough to kill an average human being.
Of course, 17 year-old kids think they are invincible, but it won't hurt to make sure your kid knows about this. If your kid is male, you can also drop the fact that nicotine is known to cause erectile disfunction. I don't know if this still true, but when I was 17, impotence was way more worrying than other, actually more serious health problems.
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Sources:
Wong HP, Yu L, Lam EK, Tai EK, Wu WK, Cho CH (June 2007). "Nicotine promotes colon tumor growth and angiogenesis through beta-adrenergic activation". Toxicol. Sci. 97 (2): 279–87 – Koldito Mar 12 '14 at 8:32
Natori T, Sata M, Washida M, Hirata Y, Nagai R, Makuuchi M (October 2003). "Nicotine enhances neovascularization and promotes tumor growth". Mol. Cells 16 (2): 143–6. – Koldito Mar 12 '14 at 8:33
Davis R, Rizwani W, Banerjee S, et al. (2009). "Nicotine promotes tumor growth and metastasis in mouse models of lung cancer". In Pao, William. PLoS ONE 4 (10): e7524. And many more like these – Koldito Mar 12 '14 at 8:35