If I were you, I would tell her in the nicest way that you are afraid of her teaching him anything bad and undoing the training you have so far done with him, or if you don't want to be too direct, say that you are afraid of her getting hurt while you are away or that you were planning on giving him a little rest break.
I would allow people to ride my horse, but only if I was around to watch them. I am very fussy with that sort of thing, and also since it is your horse and if something happens to her (even if she tells you that it's not your fault if anything happens to her) I'm darn sure her parents will not see it the same way and will hold you responsible.
Otherwise as meantioned, get someone else that you can pay to look after your horse. That way there is no risk involved.