Unfortanetly I think she is going to have to chalk this one up to a case of lesson learned. If you had nothing in writing, you lucky to have gotten a single cent back because he could have just denied ever saying that. Not to be the spoil-sport but I can't see you getting anymore money back from him.. harsh but true. You can try, but I don't see it happening - here's hoping you prove me wrong.
Next time, go closer to home, view the horse, try the horse, take it on trial if you can and make sure it is what you want. You will find yourself a perfectly good horse for a fraction of that price.
Good luck with everything :)
Stop for a minute, open your mind, learn. You may not agree with what I say, I may not agree with what you say but we will both learn something new.