@boomerang,
Has he had a sleepover yet?
If no, that'd probably be the place to start -- have a trial run (or three) where you CAN just drive the kid home if things go south.
I'd be scared to try it I think, but really depends on the friend in question.
Talk to the kid's parents too, in a private, non-hope-getting-up way, to see what they think... they'd be able to best advise re: their kid's abilities.