Are you sure [he admitted to smoking marijuana over 20 years ago] part? Unless he was admitted to prison due to breaking federal law 20 years ago, which may leave a criminal record in system, I don't see how he can get a [life time] ban. CBP can only refuse entry if they found you are [in possession] of marijuana, or they deemed your behaviour unacceptable at the interview. To get a life time ban, if your source is correct, needs a much bigger offense. For example, the guy served a prison sentence for crimes related to marijuana 20 years ago and lied about it when attempt to enter US. That would be the only plausible explanation IMHO what could have happened.
I would urge you to read the story again, or at least attempt to post a link to the full story. Smoking marijuana 20 years ago is very different from [got caught and left a criminal record from smoking marijuana 20 years ago].