Then I'd be unhappy. If there's still parts of round bales out there he's probably fine, but they need to be able to eat some kind of grass, hay, roughage, not just grain. Without knowing what the pasture actually looks like I can't really say if I think he's being a slug or if you're maybe overreacting. I can't imagine (since I run a barn myself) telling one of my boarders that they were over reacting, I might ask if they'd noticed that I'd carried small squares out or that there was a partial round still out there, but ...... Well, anyway, I try to be a little tactful.
On the odd occasion that my horses have eaten up their round bales and I can't get one right away, I do carry small squares out and set them around the pasture to tide the horses over until the hay guy can deliver more of the big rounds.