I'm going to play the devil's advocate for a second...because I know how it can be to be the one accused when you haven't actually done anything wrong, just because the accuser didn't know the whole situation.
First, a lot of horses are lazy...so the fact that the horse didn't want to trot doesn't necessarily mean he was "exhausted." He might have been tired, but that's not really a dangerous situation. Second, just being able to see ribs doesn't always mean the animal(s) are neglected. Some horses have trouble keeping weight on or are built in such a way that their ribs show. If every other horse appeared well-fed, I wouldn't be concerned. If they all looked underweight, then you might have a problem. Third, an hour long, leisurely trail ride doesn't always necessitate a rest for the horses. I often work my horse for an hour--most of it at either a jog or a lope, working on various maneuvers or skills (never just "leisurely" riding along...he's working hard to stay correct at all times)--and I don't usually stop to give him a break. If the horse is in shape (and if he's been doing a lot of trail rides, he is) and all it's doing is walking or trotting on a trail, he doesn't need to stop. Fourth, unless the horse is extremely hot, there's no reason they have to be sprayed down/cooled off. We all sweat when we get hot, and we might enjoy a cool shower, but we don't have to have it. Horses often like a cold bath after a hot ride, but unless the horse is really hot (lathered/covered in sweat, blowing, etc.), he probably didn't need it. Fifth, it's more convenient for them to keep the horses out and together (instead of having to go back inside to get another horse), and there's nothing wrong with them being outside...plenty of horses are kept outside 24/7. Often, it's cooler/better ventilated outside than in a barn.
It's possible that the barn doesn't treat their horses well, but from the sounds of it, they're meeting the basic needs of the horses...if it isn't the life of luxury, it isn't bad enough that anyone would officially do anything about it, or even that they should. If the horses are truly being overworked (so that they're exhausted and overheated) or underfed, then something needs to be done, but if they're just in the same old routine day after day with no luxuries, they're not suffering. At worst, some of them will probably become sour from doing the same thing every day without a break, and hopefully the stable does keep an eye out for that and will rotate horses out if necessary.