I'm sorry for your loss :hugs
It looks like she had a pendulous crop. Interesting that is was air filled though.
If you don't mind, did the vet take swabs and blood samples? Any results from those?
https://poultrykeeper.com/digestive-system-problems/pendulous-crop/
Do you have any photos?
How old is she?
What breed?
Does she lay eggs?
Any bloat/swelling or fluid in the abdomen?
When did you check the crop - after she was eating/drinking or first thing this morning?
Is the chicken acting sick at all at this time?
You mention your friend said they saw this...