if it's *just* a county fair, she would probably be able to get away with a hackamore...or bosal. (unless they have jr. Horse classes or something like that to categorize the age groups)
Unfortuneately, she would probably get dq'd for a halter+reins.
Have her check the rules on the show bill - they are usually pretty straight forward (and if theres no rule against it, in my book it's allowed :) )