I'm in the pay for the dead horse pond. It's sad, but such as it is. The agreement was made. Unless there is some kind of proof that the horse had a pre-existing health condition that the seller knew about, the purchaser is obligated to pay.
If the purchaser had paid for the horse in full a month ago, rather than on installments, I doubt anyone would even be asking the question.
OTOH, if I was the seller, I'd feel really bad for the purchaser and try to help. Maybe by helping to find another horse, or forgiving the debt (since it's relatively small), or at least delaying the requirement to pay. I'd want to do something for the purchaser.