Looks like a dupe of this thread has been moved to the medical area, but this is the thread that has the only responses, so I'll do mine here. If you don't find it here later, though, check the medical area.

Blood testing is a perfectly legitimate way to screen for cannabis, and I'm guessing your friend's parents may well ask for an entire drug screen with that blood test instead of just a cannabis screen. It's easy enough for the lab to check for everything while they have the sample. If he's already laid off weed for one month and has another month before the test, I'm sure he'll be fine. Hair samples tend to reflect cannabis use for much longer (up to a year--but they only take the most recent three months' growth for the hair sample, generally) than blood or urine.

Give your friend this link so he can read about various testing methods and results--and what the timesframes for positive results are. Look for the drug testing link at the bottom of the page. This is a good site that everyone ought to have bookmarked. Full of good, unbiased info.
http://www.erowid.org/psychoactives/psychoactives.shtml