From what she has said, she has pulled all sorts of stuff behind him. It is the Shafts that freak him out(correct?)
You could try a shaft trainer(basically the same thing as the indian shafts above only they don't extend down to drag along the ground). The shafts make .a horse very claustrophobic because they are constant. Your pony has learned if he bolts forward, he can escape being trapped between two scary poles. If he turns, they are there. If he walks off, they are there. http://www.drivingincolor.com/training.html
I prefer the wood design of my father because it does put a little weight on it and it doesn't
Keep in mind that driving is more dangerous than riding, pretty much because of the cart. If you get bounced out, or if the cart flips, or if you hook him too close(or the cart shifts foreward), or if he gets the reins caught under his tail, there is a higer chance of it ending in disaster or even the death of your horse. (seen it all)
A horse a the World Championships died this year in a class supposedly because they hooked him too close.