She should way more than 800# at 15.2 H - poor thing. Breed her? Oh my god!

Are you prepared to take care of her even if she will never be rideable? Usually you can treat a displaced hip- but it's expensive & a long road. There is usually a fracture involved that results in the displacement along with sacroiliac issues. If treated & healed correctly, they can make a somewhat recovery. I doubt this mare was even ex-rayed or on stall rest. By now, new bone has probably fused so she is healed as much as she is going to get. You can always have your vet ex-ray her and do a pre-purchase to see what issues you will have on your hands.