That's not her stomach, but the cloaca is where the large intestine and reproductive tract come together. In a cloacal or vent prolapse the intestines can come out, but there doesn't seem to be that much hanging out. Does the tissue still look dark? It can be hard to push the prolapsed tissue back inside of it dries out or there is swelling. The longer it stays out, the harder it gets to push back inside. Chickens can be pretty stoic about showing pain. I hope you can save her, but many don't recover, and prolapse can recur in the future.