I doubt they have enough cognitve ability to handle abstact concepts like death. They're more like babies with no understanding of object permanence. For example, if you take a hen's chicks for a few days she might not recognize them as hers if you return them.
There will be some renegotiating of the pecking order when she starts roosting with the flock again. While she's raising chicks she'll stay with them on the ground. She'll be considered flock adjacent, but the roosters shouldn't bother her because she's not laying and the hens will ignore her because she's minding her own business.
We can’t tell if chickens recognize her or not, recognize death or not, or what the span of memory is in a chicken, but irregardless they will treat her as a new chicken, in terms of flock hierarchy. Maybe reintegration goes quicker or smoother with a hen who has been in a flock before rather than a completely new one? I’m not sure. Just expect hierarchy pecking and posturing.
I disagree with that statement that chickens do not understand death I've had chick's died and their siblings have become very sad and have stop eating specially if they were always together also remember reading an article in psychology and chickens. I also suppose it depends on breed and raising situations.