I think that's a kind sentiment, but the counter arguments would be (1) they could very likely overdose and die, and (2) an addiction changes who you are, so it's not that "they" want it, it's the addiction that wants it. Your friend is the "they" before the addiction, and likely hates themselves after every time they fall off the wagon. To be clear we don't know the exact details, so we're speculating ; I could imagine some universe where it might make sense to let them, but in most cases I think what the real "they" would want is help breaking the addiction so they can spend their time doing the things that actually matter to them and give their life meaning.

That sounds incredibly patronizing. For some people, just being comfortable and happy for the remainder of their brief miserable life is meaning enough.

This video popped up on my YouTube recently and illustrates this idea well: https://youtu.be/JyzJrkF21MM