What would you do if you found this out?
Perhaps this is just my autistic brain, but I'd try and distance myself as much as possible from the person.
Well, it is to me. Go google autism.
Yeah, no, you don't just say something completely judgmental like that, in
full knowledge that your autism is probably influencing your viewpoint on it, then try and shrug it off with "go google autism". You're not getting out of jail that easy.
Most people know someone who is, or has been, a drug addict. Because drug addicts are actually really capable of being fully functional, completely "normal"-looking people who bury their issue because of exactly this sort of thing. Drug addicts are not reckless, immature, uncaring, or in general automatically bad people. Addiction is a serious issue. It does not discriminate, it is not fun, it is not easy, and most importantly it is never asked for.
Shrugging off all drug addicts because you don't understand, and have made no attempt to understand, their issue is incredibly shortsighted. Think about the reasons why someone would choose to become an addict, realise that there are none, and then reconsider what you'd do if someone close to you revealed that they had issues with addiction.
Addressing the issue at hand, it depends on what they want. If they want help, I'll give them it. If they don't, I'll give them whatever they do need. It's never a simple issue and I'd need more specifics to answer the question properly.