Dust shouldn't be too big an issue as they're not inhaling a mist, they're inhaling the evaporated fumes.
As far as health goes, it would depend much more on the quality of ingredients they're getting. I'm not talking about microbiologic or viral issues either, I'm talking like the ppm of heavy metals contained in the syrup. Lab cleanliness certainly is important, but not as important as the purity of the starting ingredients.
Almost all e-juice on the market is USP grade. If you were worried about contaminants, just by using reagent grade or higher and a cheap DIY glove box with basic sterilizing equipment, you've already exceeded the cleanliness of every commercial e-juice on the market that I've seen. After that it's just a matter of following proper ISO protocols in regard to laboratory cleanliness and how much money you want to spend. Cheapest option for a sterile e-juice would be to build a large positive pressure glove box and use only sterile pre packaged tools and sterilization and depyrogenation of beakers/etc. all of which are far, far beyond necessary for making a liquid you're are then going to deliver by dumping into an open-air vial and then super heating to the point of vaporization + inhaling.
More than necessary, but not, in anyway, unobtainable for a home laboratory.
So the answer is "It depends". Are all DIYers going to be cleaner than all manufacturers? No. Are all manufacturers going to be cleaner than all DIYers? Also no. Is your friend's DIY juice 'good enough' to use? I would ask to watch him while he makes it, take note of his lab practices and chemical purity levels, and then make my decision based on how much his lab resembles the cleanliness standards of your average meth lab.
Really though, if he's at least following basic food safety protocols, and not using some below-food-grade chemicals to make his e-juice you're probably not going to die if you take a hit.