This happens all too often! I don't know what is wrong with people. How uncaring does one have to be to not NOTICE that there is a problem? I have always wondered this. Even if one knows nothing about horses or how to care for them, once the halter gets to the point where it's cutting into the flesh and causing open, festering sores....how can they not notice? Are they just so stupid, or just so uncaring that they don't even LOOK at the horse once in awhile?
I remember one time our vet was here and he was telling us how just the day before he'd been at a place where he had to cut the halters off of two horses--they were in much the same shape as your little guy--and he was some angry with that owner. He said he really wanted to file charges against the person, but he didn't because as he said "you can't prosecute people for being stupid". It wasn't deliberate neglect/cruelty, the person simply didn't know anything about horses and hadn't a clue. The law at the time didn't provide much for a situation like that--that was quite a few years ago now. I hope he said enough to the owner that the guy wouldn't let the same thing happen again--and I'm sure he did.
I'm sure glad you got the horses back and this little guy is healing up well.