I don't know. I wouldn't be able to give you that kind of advice without knowing exactly what's going on and knowing the extent of the abuse. There is no doubt in my mind that it IS abuse though.
It is ultimately up to you if you report her or not but I would definitely move out before doing it. You don't want that to blow up when you're there.
Other things to think about
Are the horses thin?
Are they living in bad conditions? (i am assuming no to this answer since you board there)
Do they have any open sores as a result of being chained up?
If none of that is applicable, I don't know that the animal control people could do anything. But then again, I don't have any experience with animal control