Chickens get all kinds of cancers. Reproductive cancers, and tumors caused by Marek's disease and Lymphoid leukosis being probably the most common. If you google "cancers in chickens" you will get some links. I think imaging at least, would be helpful, to know what you are dealing with. I'm...
The bird that passed, that looks like necrotic tissue. Other bird, looks like it's going that way. I would call the vet, see if there is a report from the bird that was necropsied. Vet care is probably your best option at this point. If metal was ingested, that would show up on xray. If...
Definitely pictures.
Even if the vet retired, the practice likely still has records, I would call and ask if they do.
Initial thought, ingesting something that was hard/sharp which stuck in the crop or just below that may have caused a puncture, blocking digestion, and becoming necrotic from...