Oh, that hurts. We have been there, my family and I. We are true birders so that makes it even worse. The nestlings are so frail, and you're right many of them don't even live to leave the nest of natural causes, but this one was special.
You tried your best to give her the chance. I can only say that if Dan didn't find her in the first place, she wouldn't have lived even those few more days.
Last year we had a fawn born in our field, we watched him for a week every day with his mom, one morning we awoke to Black-headed Vultures in the yard - seems something had gotten to "little guy" in the night, or perhaps he died of natural causes but for three days the mother deer came out of the woods every hour to the spot where my husband had removed the carcas. I can't tell you how this broke our hearts ... and to this day my 11-year-old daughter does not know what happened. She thinks the mom just took him to another field.
SIGH - sorry to go on and on here.
anyway, my heart goes out to you!