I also would not waste emotional energy or time fighting with your son over the obvious problems with the girlfriend and creating a divide that does not exist. If he cannot figure out how she is manipulating him against you, and why (attention creation disorder), then he better learn fast. Probably you are already kicking yourself over this, but why did you let a teenage girl who was a recent girlfriend, move into your house in the first placed? You need to take ownership as to how you were maneuvered into creating this too, which you alluded to with how happy you were that he was for the first time interested in a "horsey girl" and that you have come to depend on him to take care of your horses now, these are your horses and not his right? Think about that please, and he will become less of a part of this problem. He is still a teenager himself isn't he, is he in school or working outside the home?