I'd get stoned as a monkey the day before the test, purely on principle. Well, not purely.
It's nothing to do with my work what i do when at home. If i'm not drunk/stoned, and able to do my work, i'm fulfilling my obligations.
I don't smoke, so perhaps i should start. Smokers get extra breaks in which to take their drug of choice. I'm not a big drinker, but i've seen plenty of people hungover at work. When was the last time someone's work suffered because they were stoned the previous night?
My current employer has no such policy, but if they did introduce it, i'd be more than happy to leave. I wouldn't work for someone who thought they had the right to tell me what i could and couldn't do with my wages/spare time.
As for your brother, it's not his fault that the employer is insane. If he's getting interviews, and he has qualifications, he'll get more interviews, so tell him to hang on in there. It can be tough when you're looking for a job. The last thing he needs is family kicking him when he's down.
I obviously don't know the circumstances in which you're considering giving him the elbow, but he's family ffs. Treat him as such.