Depending on what State this occurred in, you may have a valid lawsuit for negligence. The owner and trainer have an obligation not to allow a rider on a horse unless that rider is experienced enough to handle a horse of that temperament. While there is some assumption of risk in activities of a recreational nature, the owner of the premises is obligated to ensure that the activity is no more dangerous than it appears to be. An unbroken horse with a history of throwing off riders should not be made available to a novice rider.
On the other hand, since you say that your daughter was working at the barn at the time, if the owner of the horse was also her employer, the workers compensation law may bar her from bringing suit and she will be limited to workers comp benefits.
If you need more information please provide more specifics, such as the State where this happened; whether your daughter was the employee of the horse owner; and whether she received any workers compensation benefits.
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