I would suggest making it as adjustable as possible, so that it can fit more than one horse. (you did say she had multiple horses to ride)
in the pics at the site you provided, there are adjustment buckles under the tail, across the shoulders, at the crupper, and everywhere it attaches to the saddle. notice how it attaches to the saddle at the back and at the sides. make sure she has a saddle available that you can attach to properly. also I would suggest making the crupper removable so that it can be taken off of any horse that won't accept it.
you can buy the crupper part already made, but if you are going to make it yourself i would suggest looking at several up close, to see how they are made for comfort. or maybe your friend already has a crupper that you can use.
one more suggestion that i'm sure your friend already knows...don't wait until the day of the event to put it on the horse for the first time. give the horse time to get accustomed to the feel of it while moving.
that's a beautiful rig, yet fairly simple in design. i think it would look stunning on the field or at the faire.
good luck, please show it when you have it finished.
ken