First, she learned not to approach his stall, then she learned not to enter his stall even if the door was open. Now, she's learning not to approach or enter the paddock...
http://www.youtube.com/watch?v=Di3cH2T6sHc&feature=player_embedded
As you can see, the training commences with an already exercised dog, tongue lolling out. To take a keen dog out for this exercise, initially, would not be as good at setting the dog up for success. With continued training, she will progress to easily being able to ignore the horse in the paddock when she is keen.