You've taken careful notes and made adjustments along the way, but you've probably answered your own question. The body has only so much energy to spread around between athletics and immune response, so a whole truckload of gummy bears would probably not have helped her to push any harder.
Any time I hear that somebody is looking to set a PR, I can safely assume that two things are around the corner: overtraining and weakening of the immune system. Either could produce the results you describe, and both together almost surely will. The elevated temperature, labored breathing, and lack of the big boost in performance you were anticipating, are to be expected.
So rest up, don't be too hard on yourselves for being human, and live to fight another day. Maybe next time out will deliver the numbers you are looking for, but patience is key. Good luck!