It is very easy to tell how young you are and how inexperienced you are with life. You are just not willing to let this go, you want to prove to everyone around you that instead of listening to adults with alot more experience with horses and life in general, you want to have the last word and keep this going.
I can teach a dog to stand still like a statue in the show ring, but when its outside waiting to go in, if it moves its feet, thats okay. The dog knows the difference when he can move and when he can't. A horse would be the same. They do have understanding of training.
It is her horse and her rules, not yours. If she insists on the horse standing still for grooming but doesn't mind him moving around for being tacked up, then who are you to decide what is right?