Provide documents that support the value of the horse. In this case, J.B. would need to prove the value of the 12 year old mare with which she parted in exchange for the 8 year old mare. Possibly, she can testify about the value of her own horse. Or, especially if J.B. is not experienced with horses and their values, she should consider hiring an appraiser or industry professional to report on the mare's value. Be organized and succinct at the hearing when you make your case.
Few judges have experience with horses.
Certainly, J.B. can hire a lawyer to do any number of things. The lawyer can advise her of her rights based on her state's law. The lawyer could file suit against the horse trader on her behalf. The lawyer could write a threatening letter demanding that the horse trader resolve the matter. The problem is, J.B.'s legal fees might just exceed the amount at stake.