So... Your BO just took the liberty to tell this woman she could take YOUR horse for a trial? Did she run that past you first??? The only person that has any business letting someone try out a horse is the owner. I would be mad as hell if someone did that to me. I had that happen once, actually.
You have all the authority in the world in regard to your horse. What you say goes. Period. If you aren't comfortable with something, put a stop to it. If you don't want to sell your horse, don't! That's no one else's business. It sounds like the woman and her daughter just aren't the right people to lease him. (Sorry if I repeated anything. I didn't read through all the posts).