Trespassing? Interference? I don't really know, but that is not the point.
Do not mess with this horse. It is not yours, and unless it is in immanent danger, (ie colicking, or cast), you should not go against the owner's express instructions.
If you have honest concerns about the horse's well being, then talk to the barn owner or call the humane society if there are grounds for it.
If you do mess with the horse and something happens, you are liable.
Not everyone treats their horses the same, but it is not illegal to leave it in a stall if the horse has food water and shelter. You are not there 24/7, so you may not know how often the owner may come out and exercise her horse. Hearsay is not necessarily fact.