As long as you have access to the horse and the horse was not stolen, and you are free to move the horse out of the new barn at any time, again; without a written agreement that the horse NOT be moved without your knowledge or consent, there is no legal recourse. Now, if the horse sustained an injury or illness directly related to the move you might be able to do something but otherwise it is between you and the trainer at this point.
I agree with Verona..while the horse is still up for sale, and if you decide to remain with that trainer, I would get a very clear contract into place. I will say that I question the trainer's, uhm, professionalism if he could allow a family member/girlfriend whatever to "force" him to move a horse he doesn't own. That excuse sounds a little strange. Personally, I'd be looking for a different trainer if possible.
Just out of curiosity, and I missed it in the first post...were you contacted about the move and refused or was the horse just moved? If you were contacted first, refused to have the horse moved and the horse was moved anyway you may have a legal right as technically the horse could be considered stolen. If the horse was just moved and then you were notified, then it is more a case of really unprofessional conduct. Even if you were previously notified the case is still very weak due to the argument that you know where the horse is located and have free access and can move the horse at any time.