Sky, that sounds like me :p If my mom goes in to clean one of my stalls ... I have to go in after her, and sift through all the shavings she didn't, get the hidden pee spots out, LEVEL IT ... I just thank her for the help, and ask her to fill the easy water buckets instead :p
I agree with getting a contract for sure. If something pops up, the court wants it all in writing. I would get a whiteboard, and write on it a list of things to do for that day. Treat this lady like an employee, because in all reality, she is.
Were you planning to hire instructors and give lessons? Lesson horses? Arenas? Just curious :)