I can see why the women would be frustrated. However, if she hasn't paid for the horse in full then I would think she would be a little more tolerant. After all you are working with her as well. I wouldn't cancel the sale over of this.
Explain your situation again, set a date that you can realistically transport him, and as the others said make sure it happens. If she wants him sooner than that, tell her then she will have to arrange suitable transportation.