You're implying addiction, but in the more common case, people with ADHD can be apprehensive about getting off the meds for the same reason people with chronic pain can't easily get off painkillers, or people in general can't easily get off oxygen.
> like my friend N., who rather than swallowing his medication (Ritalin?), he crushed it & snorted it like cocaine
That's just drug use, not ADHD treatment. I wonder what your friend's dose was? Snorting instead of swallowing is massively increasing the absorption rate, but even then, I don't think you can get high like this, if you just do your regular regimen through the nose. But doing a week or month's worth of pills this way at once, I guess sure?
I don't have the data handy, but IIRC the difference in dose size between therapeutic effect and drug abuse that leads to addiction is ~100x.