Sorry to hear that; I was hoping for the diagnosis to show an easily fixed problem. I found this on Horse and Hound, obviously the recovery rate is going to vary based on severity, but for what it's worth: What are the chances of a full recovery from a pelvic fracture?
The treatment of pelvic fractures
invariably consists of two to three months of box rest
followed by a gradual return to work.
If the fracture
is displaced, or likely to displace, it is prudent to cross-tie the horse to prevent it lying down, as the fracture
may worsen if the horse has to struggle back to its feet.
In many cases, it is sensible to give the horse some painkillers initially to make it more comfortable.
The prognosis of pelvic fractures
depends on the position and the extent of the injury. Generally, injuries that are displaced have a worse prognosis. Furthermore, injuries that affect the hip joint often do poorly as a result of subsequent osteoarthritis
Occasionally, displaced pelvic fractures
are severe enough to warrant a horse being put down
immediately, although this is rare.
Many horses with pelvic fractures
make a good recovery and return to the level of work they were doing before the injury.