my first horse had an injured suspensory check ligament and it took at least 2.5 years off work before i could ride her (id lent her to a friend who galloped her when she wasnt fit) she was never badly lame but just couldnt come sound so its a longterm injury.
i also had a "posh" friend who had a tb on full livery cos it looked "the part", she could only ride the horse if it was semi-fit, when the horse was fit and healthy she couldnt manage it. this friend rode my other friends cob a few times and loved him thought he was great fun but would never own something "common" like a cob when actually her was perfect for her.
she needs to recognize what type of horse she needs, she may still be able to work with a different rescue/rehabilitated horse with success as there are plenty of ponies out there but u need to find the right one.
selling a horse like that in this market would be really hard, i dont know if a rehoming centre would swop him for something more suitable?