As for the FDA part, there are a ton of studies out there that either used ......ed testing conditions (One hooked up a CE5 to a battery (didn't say the voltage) and let it run continuously in a vacuum til the entire tank was dry or the coil popped...) and/or sub par equipment (The one I saw that claimed to find heavy metals in "popular e-liquid" vapor turned out to be some no-name brand from China). As for the nicotine, you might wanna remind him that it's in a LOT of foods he probably eats often, like potatoes or tomatoes. Then break it down and explain how there's no actual combustion that releases tar and other carcinogens, and how nicotine is similar to caffeine (hence the warning lables). If he's still trying to argue with you, I'd just laugh at him.
Rule of thumb tho... Ignorance can be fixed. Stupidity is permanent.