Unfortunately, you found the problem too late to help or be able to fix. If it were a newly hatched chick or you identified the slipped tendon when it first happened there are ways to repair it, but now that the swelling and inflammation have set in and at its age, it is not possible.
Chicks can find a hundred ways to kill and maim themselves let alone accidental mishandling. I know the awful feeling of causing a wing or leg to be injured, it is just life. Garden Peas had to give up all of her birds and brought them to me including the crippled bird, after all that she went through to try to fix the bird nothing worked and I had to put it down. They can live a long time but being confined to the ground brings a lot of unhealthy complications.