It's not your fault, I guarantee he'll smoke without you, it's his choice, you didn't force him, don't feel guilty, get on with your life, don't let it affect you.
Exactly, if OP weren't the first person to teach him to smoke, then out of curiosity and really wanting to do it, he will smoke in a different way, so OP is not at fault here. OP shouldn't care what drunks and addicts say because their minds aren't clear, and I believe they don't even know what they're talking about until they're sober. But it would be great if OP could take him to rehab and get his sanity back. It was a good thing to do since the two of them were once best friends.