Anti-drug GAF out in full force, I see. While you are right to tell the guy to stay the hell away from heroin, don't go off spouting saying he'll be hooked by tomorrow or that he's a fucking moron for being curious. Smoking and snorting heroin are two of the least habit forming methods of using heroin, while injecting it is the most (obviously), and are, in fact, quite manageable. Doing it once won't hurt him, and it's pretty clear that he understands the powerful effects of the drug. No need to get all vindictive on the guy for being curious.
I used heroin for four months and quit one day without even thinking about it. It's entirely possible, and depending on how you, here's that term again, moderate your use, you can use heroin safely and not become dependent on it. I have an INCREDIBLY addictive personality, I was an alcoholic at the age of 18, and I was obese by the time I was 15, because I loved soda and candy so much. Heroin was the last drug I tried, after going through weed, E, coke, LSD, mushrooms, all of that, and it was by far the easiest drug for me to manage. There are pros and cons to everything in life, and you got to learn how to control that to live a meaningful life. I'm not saying everyone should go out now and try everything and see how they can manage it, but I think my point is pretty clear.
Being curious doesn't make a person stupid or flawed. Leave the kid be. He tried it once, he enjoyed it, and he's putting it aside.
That said, do stay away from it. I'm all for experimenting with as much as possible in life, but heroin is a "sometimes drug," like once or twice every six months kind of thing.