Thanks for posting your query with us.
I wish you had described the tests a bit more clearly. I'm guessing the doctor did an X ray
plus some blood tests and all came clean.
The pain in the middle back - Is it right under the shoulder bone (wing like scapula
bone) at the back?? If yes then the pain could be due to an alteration of the scapula bone. This happens because of weakness in the scapulo thoracic muscles. You can probably go to a physical therapist and get a few sessions of ultrasonic therapy
and icing to take care of the swelling.
Also how old is she?? Does she have any chest infection
as then she needs a physician right away.
You can help her strengthen the scapulothoracic muscles by finding the following exercises online.
1. Scapular Retraction
2. Shoulder Bracing
3. Shoulder push-ups standing (push on wall)
4. Shoulder External Rotation (actively)
As far as the rib cage
is concerned if there is nothing wrong with the blood tests and x rays then make her do some breathing exercises and stretching for the pectoral (chest) muscles. Also make her inflate a balloon 5-10 times a day so as to strengthen her chest and breathing muscles.
Hope this helps her.
Give my best wishes!!!