I don't know if this is any consolation, but I reckon you can't cure a sick chook about 90% of the time, and the other 10% they die anyway. As a local vet described it, they have the sick chook look. I have a couple of blokes here to dong the sick ones for me. Putting in isolation rarely is successful, just prolongs the illness, and then you have to decontaminate the isolation coop. I've never heard of sulmet, but it wouldn't hurt treating the rest of the flock with it as a preventative.
I don't know what poor Ms Purdy's illness might have been, but do watch for sickness in the others. I have heard that worms can account for a large number of unknown chicken deaths. They can sometimes have feed problems too, like sourcrop..the crop will feel big and hard. Contaminated feed can drop them very quickly. Or maybe she did have trouble with her first egg. I suppose the cause of death ever remains elusive in CCD (chicken collapse disorder).