Is she getting the Apple directly from Apple? You usually have to do this in order to get an edu discount, unless the reseller offers some discount. Apple's discount doesn't carry over to retailers/resellers.
There's no paperwork needed most of the time if you do it online via Apple's online store -- simply configure the computer normally, check the price, then log into the Educational Store (there's a link on the store page) and do the same to compare prices.
In my experience, there wasn't one, flat percentage discount -- some computers were discounted $100, others $200, but not equalling one, constant percentage. For example, with Apple's educational discount for my school, I can get a 512MB shuffle for $99 (0% discount), or a 1GB shuffle for $119 (~8% discount). Same with computers -- some discounts are bigger than others.