There's still a lot of stigma around addiction, and because of this, a lot of people don't understand how to best support their loved ones. While this applies to all addictions, the stakes raised when it comes to hard drugs like heroin, since a lower percentage of the general population has personal experience with the drug, and it has a higher death rate upon relapse than alcohol or other drugs.
That being said, one of the worst ways to express "support" for someone fighting addiction is by gossiping, making unsubstantiated accusations, and not believing them about their own recovery process.
"Context here: I'm an ex heroin addict. The why and how isn't important, but I got hooked at 15, tried to get clean at 17 but relapsed hard, finally got my act together and got off it for good at 21. I'm 26 now and completely clean. I don't even drink, except for on special occasions where I'll have two or three drinks at most."