I will probably end up leasing out my horse eventually. With my career I don't expect to be around all the time. However, they would have to be one very special leasee! My horse is my baby. I don't want all the work I've put into her being undone. I would probably insist that they have weekly or biweekly lessons with my trainer, and it certainly would be in the contract. They would also have to be open and willing to exercise my horse in all the ways that I do, liberty being the big one. I can see my horse being soured very quickly if all she did was jumping training.
As smrobs said, a very well trained horse isn't cheap. It takes many many years of training to make a good dressage horse, and very little time to undo it. The owner is protecting her investment and her horse. She's probably putting you through a trial period (I know I would!). Try to understand, I'm sure if you were in her shoes you would do the same (maybe not quite to that extent but you would be involved for sure). However, as others said, don't let yourself be pushed into extra expenses, that would not be fair. But on the other hand, if she is paying for it, why not go to the clinic? Everything is experience, good or bad! At the very least, you'll learn what not to do. At the most, you'll have a fantastic time and come out a better rider than before.
"You can do something wrong for thirty years and call yourself experienced, you can do something right for a week and experience more than someone who spent thirty years doing the wrong thing." ~WhattaTroublemaker