You can tell she's in pain because when's she's laying down she shakes. it's breaking my heart.
Leg bone deformities are common in chicks. Slipped tendon is probably overused to describe some of these, including valgus deformity.
They are very hard to successfully treat. They typically get worse with age, but it is definitely worth trying to fix. Some raise these chicks with extra TLC, and sometimes keep them safe with another one or two birds in a small pen. Here is a good help site for fixing a slipped tendon, and some reading about leg bone deformities: