The more common splay leg can be fixed with a hobble on the legs for 3-5 days a lot of the time. But the worse slipped tendon or rotated femur is much harder to fix.
I would bring in another chick to be her companion. They grow up very lonely as a single chicken, since the groups that are hatched or brooded together always seem to stay together the first year or so. It is taking a chance however since this chick may not survive for sure. Your chick may...