I'm so sorry you lost a hen. :hugs
We find glass from jars, bottles, plates etc around all of our trees after hard rains. Apparently way back before we came here there was a lot of target practice and the remnants were left to lay. So we are always picking up the shards. Some big, some...
It does look like glass that has gone through her gizzard. Seems like she's doing fine despite that. I would have a look around and try to clean up things they shouldn't eat, or fence off areas that can't be to keep them out. Glass will get sanded and rounded in the gizzard, but pieces of...