It doesn't look good either way, but how you phrase it lightens the load and doesn't make you look like as much of a dumbass, which is the point. People make mistakes, that's life, every HR department has seen it all. If he's able to go up to his new employers and prove he's clean, and he can prove it with previous performance reviews, why wouldn't they hire him? If every company looked at "what a dumbass" felons wouldn't get hired, every person with a charge, or been fired even for little reasons couldn't get hired again.

Dumbasses are the ones that try to lie about their past. Honesty is a great concept, but you can't let it hurt you at the same time. You are honest about it, but keep it simple and sweet, and if they ask you to elaborate, just say I smoked marijuana once, I made a mistake." Shows you are responsible for your actions, which in turn, makes you a valued employee. Never bring up information they don't ask for. If they don't ask, don't tell.

This isn't a guy who worked there for 3 months and got fired. He had 20 years of good service before 1 incident.

So laugh, but let me ask, have you ever been in this incident?