Anything can happen in a race. If for some reason you lose the race, you have to let him ride your bike. A bet's a bet. Most important question now is...what is his level of experience riding bikes? My rule of thumb is I NEVER let anyone ride my bike, unless it's absolutely necessary, like some kind of emergency. If he rides it, all it takes is a patch of sand or whatever, and he drops it...you know the rest.
And since he's a nag, that would REALLY s*ck to have HIM of all people drop it! Just my .02.