I agree with cake mom. If she signs a waiver and please make sure her parents sign a waiver since she is a minor, then make certain hours that she can come. I am not sure why guests would be coming with her, but make hours for that also.
I can understand your dads feelings, once you allow someone other than your own family to board at your home/barn, then you are opening it up to the "public" so to speak and all that entails. Stopping by "just this once" early or late, showing aunt mable her horse, riding early, or making it way to easy to leave you with all the barn work and feeding because "she is busy".
I also think it might be a good way to ruin a friendship between you and your friend, sometimes things like this will put stress on you and your family.
While I understand the good points of this, I really understand your dad. When friends wanted to put their horses in our pasture here at our home, we decided it was not a good idea, you always have to worry about doors being left open, gates opened and when the friend beings a friend or two, you have alot of strangers at your home.