She sounds like a smart momma, even though it's unfortunate about the remaining egg.
Just out of curiosity, do you have an incubator to use as a backup?
Too late now but that egg may have still been good. I've had them leave eggs to take care of chicks and I put the eggs inside in a home made incubator and they hatched! Congrats on the peeps!
I checked the egg the next morning and it was not fertile with a liquid yolk and white. I had suspected that would be the case as, a couple days after I gave the eggs to Arrowroot, I checked three similar eggs; two were definitely fertile and one wasn't.
I don't have an incubator but might make a homemade one in the future for similar situations.
I could have candled the eggs but I had chosen ones with the darkest brown shells.