I don’t know if I can offer much help or not. Most prolapses are in hens of laying age, or in baby chicks which may be birth defects or seen during straining to poop. I would think that hers is a birth defect. Are you positive that she is a pullet? I cannot see anything that looks cockerel, but just wanted to ask since the tail feathers are long. I don’t know if a vet might be able to put a stitch in the vent, just to keep it from popping out, but then there might be a problem with constipation. I think you may have to just see how it goes. When she starts laying, maybe the vent opening will get more stretched. I just haven’t seen this particular problem, though. You might try giving her some wet feed to get more fluids into her.