Wow. That is SO not right.
OP, I just sent them an email. I was polite, but I said that the horse is clearly [edit] in need of immediate care, that he should be either put down or released to you, etc. Hopefully if enough people do so, they will do the right thing.
Jingling like crazy for you and Max! What a gorgeous hunk of horseflesh he was back in the day! I really hope he can come home to you. And I would definitely be going after the girl who claimed she would give him a great retirement and then abandoned him!