WHAT DO I DOOOOO!? Horse/girl trouble!?

If your friends grandmother is a horsey type person she will respect you and like you more for caring about the safety of your horse. Let this girl ride your horse under careful supervision and give her a set of rules. If any rule is broken demand that she get off the horse. If she refuses enlist the help of your friends grandmother. You shouldn't have to sacrifice the well being of your horse to satisfy the urges of one little brat. And keep a careful eye on your horse and your horses things. She might resort to property damage after this, if possible lock them up whenever your not using them. I wish you luck on your trip.