The horse belongs to you
. You have every right to watch her work with your horse. Especially if you don't know this person well, whether approved by your trainer or not. I wouldn't trust her alone with my horse until I'd seen for myself that she met my standards.
But that's just me. I'm kind of a lunatic when it comes to other people touching my horses. Take it with a grain of salt.
Hope everything works out for you!