No. Nicotine is a carcinogen. Forgive my surprise, but I'm absolutely blown away that this isn't common knowledge in 2017.
Edit: I appreciate the distinction that folks are making as far as what constitutes a carcinogen. Yes, the science we have today falls short of us classifying nicotine as a 'complete carcinogen'. If that makes you sleep better, then ok, but the science shows carcinogenicity in vitro, and substantial ability to promote tumor growth, metastasis, and recurrence in vivo .
Your friend is also potentially exposing himself and those around him to formaldehyde and diacetyl and so on, which is present in many e-cigarette liquids.
Folks: eat healthy, get a bit of exercise, read good books, and most of all, develop relationships with people you truly trust. If you find yourself needing to rely on smoking or drinking to get you through the week, take a step back and garbage-collect your life and decide if you're living the life you want, or if it's a life that's in need of radical restructuring.
(36MB pdf) Page 113 https://www.surgeongeneral.gov/library/reports/50-years-of-p...
> Nicotine is a highly bioactive compound with effects ranging from being a natural pesticide in tobacco leaves to causing addiction in tobacco users. For cancer, there is some biological basis for proposing that nicotine may promote cancer based on experimental studies that have limitations in replicating human exposure and on mechanistic studies, but human evidence is lacking (Lee et al. 2005, 2012; Dasgupta and Chellappan 2006; Zheng et al. 2007; Catassi et al. 2008; Chen et al. 2008b, 2010; Egleton et al. 2008).
(It's a couple of pages, too long to paste here. But all the people posting wikipedia links might also want to read it.)
You do yourself a disservice by so stridently and repeatedly claiming "Nicotine is a carcinogen." There are plenty of compounds we're exposed to through tobacco use that are uncontroversially carcinogenic that you could point to instead (if you feel the need to name them explicitly), as well as a lot of other nastiness like formaldehyde as you point out above. Dwelling and defending this weaker position makes it easier for uncharitable readers to dismiss you.
> Although there is insufficient evidence to classify nicotine as a carcinogen
I guess you better fix Wikipedia...
in addition to what i posted above, this paper is a good summary of what we know at this point: doi:10.1038/nrc3725