I am so sorry for your horses. I would start free feeding them grass hay, and throw them a couple flakes of alfalfa in the morning and a couple at night. The best thing to do if you can is weigh your food and your horses, to make sure that you aren't under feeding them. As for the grain, I would keep them at whatever level they were getting before you got back home, and just see how much upping their hay helps them out. I would definitely lay of the exercise until they gain some more weight especially if they haven't been doing anything to date.
I had something similar happen a few years ago, I was gone for a week, and had a friend who knows horses feed and take care of my ONE horse while I was gone, and when I got back a week later, the plywood around his stall (we had a goat) was chewed down as close to the rail as possible, and his 100 gallon water trough was completely bone dry, and as I was feeding pellets at the time, we had half a bag of pellets that was opened when we left, and several other unopened bags, and when we got back, there was no new opened bag, and there was still food left in the bag that we had opened before we left. My friend (no longer my friend) claimed that she filled up the water thing the night before, which she obviously couldn't have done, otherwise there still would have been water in the 100 gallon tank. A horse doesn't drink that much water in a day, and that she had fed. I personally don't believe that she fed much food at all, nor did she water my horse as she was supposed to. Made me so upset when I got home. I'm just glad that you were home when the humane society came by, so you could talk with them, and you didn't end up coming home to find out that your horses had been seized. If you can, I would press charges against your neighbor, or at least in some way hold them accountable for what they did while you were gone. Best of luck to you.