I had somewhat of the same problem with a pony, as well as with a horse I rode a couple of years ago. What my trainers told me to do was instead of giving the horse a lot of rein (if it's western... umm... no idea what to do), but before moving into the canter, hold the reins a bit tighter (not too tight... remember baby birds... :P ) and then ask for the canter. If the horse can't get his head down, he can't jump. So just try to keep the head up. Not SO high that the horse's movements are hindered.. but a bit higher than the withers, to right at them. Any lower and the horse is able to buck. Also, if she wants, she could find a good enough rider who can ride the horse, and when the horse acts up, make him stop immidiately. Then canter again, and when the horse bucks, stop once more. Like, a complete halt, pretty much right as he's bucking. Don't wait until he finishes. Pretty soon, he'll learn that he can't do anything if he bucks. Hopefully... :P
I just know that this is what I was told to do, and what I did, and it helped.
Also.. just work out all those jitters before getting on!