A friend of mine is doing the same thing. She is 20 and bought a green broke arab. This girl has only been riding for 4 months, and she claims that she rode when she was younger. She rides like she's only ridden for 4 months, with out any proper training lol. The horse was broke, but she fell off from 2 baby bucks at a walk, and got a compression fracture on her spine.
It happens. Though it was not the horse's fault and if you can't stay on for a little buck at a walk, you have no reason to have said horse.
Oh, and she blamed it on the horse, as well. Fail.
I really hate this. People think they are fantastic in the beggining. People really need to meet the ground, or their ER physisan many times before they become even a tad bit humble.