It actually isn't, if their vape is mis-tuned and producing ash you're inhaling it the same as you would second hand smoke - additionally the health dangers of vape fluid aren't fully understood, a lot of the chemicals that contribute to the carcinogenous natural of tobacco smoking are still present in e-cigs but at generally lower levels. The misinformation that's going around is basically "No one has had time to conduct a long term study and prove it's dangerous so it must be safe!"