I could see that she wasn't right. She was sitting away from the others, and kept moving away when any of them came to keep her company.
It looked like heatstroke, so I did a cut down version of the usual stuff. She was very warm. Chicks are very warm, but she seemed warmer than 'normal'. I thoroughly wetted a towel, with warmish water, wrapped the chick in it, then wet a small rag and put it over her head. I unwrapped her so she would feel the cooling effects, and rewet it. I put it underneath her. I checked her over to see if I could see any signs of injury, and I checked her vent in case there was blood.
I was loathe to do anything more dramatic. I wasn't sure she could cope with being more thoroughly wetted.
I squirted some water in her beak, using my smallest syringe. I mixed some Avipro ith a tiny bit of yoghurt which she ate.and then put her back outside. I put a wet towel next to her so she could lean on it if she wanted.
An hour later, I got her out again, and repeated it.
A little later she took herself up to bed.
And then I heard a scuffle from inside the coop, and I knew she had died.
Poor little poppet.