I also think that this is a "call the humane society" issue. Poor things.
The last 2 years of my TBs life I did have a LOT of trouble keeping weight on him, despite trying everything under the sun (including working closely with my vet on the issue). So he was looking pretty thin towards the end, but I wasn't riding him at that point and certainly not looking to lease him to someone else.
I also concur about horse demeanor changing once they get healthy. A friend of mine recently took on a free horse that needed TLC. Her mom was looking for a gentle, slow trail horse. They were assured that his guy fit the bill perfectly. When they tried him out, he was exactly that. Then they took him home, had his messed up teeth fixed, he put on 150 pounds, fixed his feet up etc. Gentle, slow trail horse is gone. In it's place is a beautiful, healthy, 17 year old horse that bucks, rears, spooks and mainly wants to run. My friend actually enjoys working with a horse that is a "challenge", bu this one is no fun even for her. Her mom is certainly never going to be able to ride it again.