Originally Posted by Skyseternalangel
It's your horse, and you owe them nothing. If you don't want him to go, then simply say that she needs to find another horse to show.
This. Unless you signed a lease giving them full use of your horse, you have the right to say no. That said, would keeping him in "show shape" be a bad thing in your absence?
I agreed to "share" my horse at a show one summer during high school, when a novice rider's new horse went lame. It was hard to watch someone else win ribbons on MY horse after all the training I'd put in, but it was also gratifying to see how my training/my horse helped a young rider learn to love showing.
That said, I later leased this seasoned show mare out when I went to college, and the mare was returned to be blind in one eye. While she technically could have done the same thing playing in the pasture, it happened at a show facility.