Here is a few links that might help.
https://www.backyardchickens.com/t/299473/slipped-tendon-no-cure-what-do-i-do
https://www.backyardchickens.com/t/708927/help-for-slipped-tendon
http://chickenheavenonearth.weebly.com/slipped-achilles-tendon-treatments-in-chicken-legs.html