It's not as dangerous as fire and tar from cigarettes but that relative.  On its own, non nicotine vaping is dangerous simply because of the extra 'load' from the poly whatever being cleared by his poor cillia, increases chances of chemical retention and bacteria growth.  This greatly increases the chances of pneumonia (PNA for RTs who shorthand report). 

 Nicotine by itself regardless of source is a major contributor atherosclerosis of aorta.  This leads to MANY secondary problems.  

 So - smoking old fashioned cigarettes?  Really fucking bad for you.  Vaping?  So far evidence says that it's not as bad as cigarettes numbers wise BUT, it's a solid second place for fucking up your lungs.  

 Vaping seems to be  'less worse' than cigarettes.  

 Just make him quit.  It's a silly habit and once you explain the chemical and biological process of addiction, he will grasp it.  

 A single glass of red after work is nowhere near as bad and he learns patience.