Do you keep your horses at home? If so, you will likely need to look at equine activities insurance. Also, look into a hold harmless waiver. I think you can probably download one and a lease agreement to at least use as a base from the Horse Council BC website.
Beyond that you'll need to decide what tack she can or can't use and let her know. Whether you will allow him to be taken off property at all. If he requires shoes during periods that he is being ridden, will she need to pay for a portion? Is she allowed to jump him and whether you or some supervisor needs to be there when she rides/jumps.