Yes, you always get everything in writing BEFORE you start. That's with everything, not just with horses.
I understand that you only had to be there 1-2 times a week, but that really isn't that much for free board. I worked off my board when I was a kid. I was at the barn everyday, literally. I changed school buses so I would be dropped off at the barn and I took care of 20+ horses, usually the BO would help. I wouldn't just do basic barn chores either. I would help mow, paint jumps, fencing, help with the garden. Anything that needed to be done. I'm sure the work I did exceeded my board, but I enjoyed my time there. I think you need to realize that you have it really good considering the small amount of work you do for her.
This is what happens when you have democrats in office