Like said above unless the horse doesn't have feed or water in front of it, then AC likely won't do anything.
The best you can do is try to get the owner to call a vet in and have the horse completely checked out, a thorough exam, not just a look at the eyes and ears kick the tires kind of 'exam'. I'm talking blood drawn, fecals, the works.
Do you know what and how much the horse is being fed? If so, do you know what the quality of the products are? Just because it works for a couple of horses doesn't mean it'll work for a third horse, unfortunately.
I've seen owners who think just because their one or two horses do well on 10 pounds of hay and 3 pounds of sweet feed that it should work for all of the other horses and can't figure out why their third or fourth horse is skinny. It's a shame he's not coming out to check on her.
I know you said there's no way you can talk to him about her weight, but I assume you have some way to get ahold of him? Email, text, something?