Here are some good articles to read:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7106171/
https://www.merckvetmanual.com/exotic-and-laboratory-animals/pet-birds/reproductive-diseases-of-pet-birds
Coelomitis is a newer term for egg yolk peritonitis. This is common in many hens over 2. The vet is probably guessing that she is laying internally, and there can be masses of egg material found during a necropsy (autopsy.) There is nothing you can do about this, except to give the very...