When a person has rib fractures, they are left alone unless the initial trauma caused life-threatening injuries to the lungs, diaphragm, or major blood vessels. Most punctured lungs are left alone if they stay inflated with the help of a chest-tube.
The same is true for horses. Life-threatening injuries are immediate and mostly not 'fixable'. I have never heard of a Vet doing anything other than 'wait and see' if the horse will return to usefulness.
I had a 'flail chest' in 2005 from a really severe accident. I broke every rib on the left side. I had 25 to 30 rib fractures with one broken (not cracked) in four places. I had a punctured lung that was collapsed and had a hemo-thorax. I also tore 3 ribs from my sternum and tore 3 or 4 from my spine. They pulled 2 units of blood from the chest-tube they put in. I had that chest-tube in place for 5 days, pulling blood the whole time. I also had a lacerated kidney and passed blood through my bladder for 5 days.
This wreck put me in the ICU for 14 days and in re-hab for 3 weeks, got me a $9,000.00 helicopter ride. It took 6 months to heal all of the ribs and and about 3 years for all of the torn cartilage to heal.
All of that and they did not do any surgery, did not try to move or 'set' any of the ribs and let me heal naturally. Had I bled through the chest-tube any longer than the 5 days, they would have gone in a removed that particular rib fragment and done nothing else. The first time they tried to take the chest-tube off of the pump, my lung collapsed again, so they put it back on suction. I was in a Level One Trauma Center and had one of the best trauma surgeons in the Country.
Had I had this injury on one of my yearlings, I would have probably called my long-time Vet and asked him what could be done for a suspected rib fracture. I would guess he would tell me nothing if he was not coughing or passing blood. I am not sure he would say anything different if he was. It is the kind of injury that is what it is. Only time will tell if it will affect his having a girth on him.