First, get the facts on what constitutes abuse in your state. There are no meaningful national standards - this is going to be at the state level. Figure out if the abuse law even applies to horses (it often does NOT, as horses are frequently classified as livestock), and if not, what laws *do* apply to horses. (If, frankly, any do. )
Once you know what constitutes actionable abuse, you're in a better position to document faults (and you'll also know what not to bother telling HS about. Yes the horse is skinny, no they don't care, unless there is LITERALLY NO FOOD). Again, the crappy minimal laws are all HS have to work with, so work with them as much as possible -- give them what they need to make a legal case.
Unfortunately, I have a feeling you're going to discover that no meaningful legislation exists, and the owner engaging in passive neglect until they die is perfectly legal. (UGH) In that case, your only option is to get the owner to surrender the horses/sell -- a challenge, I realize!
It's a tough situation, no easy answers. Probably the place to start is to simply nail down what, if any, state laws apply.