my youngest son, now aged 10 suffered with pneumonia aged 3 this was misdiagnosed as gastroentraitis so i was looking after him at home for nearly 2 weeks, as he was too week to even move from the settee all day I found it best to put him back into nappies, once he had been into hospital and treated he seemed to have made a full recovery but I had to go back to toilet training him again, I was told that this could have been due more to the emotional side of his illness, basically feeling so scared of what was happening to him rather than the illness itself being the cause.
Maybe this is the case with your neighbours little girl, if so you can reasssure her that with my son it took less than a week to retrain him again.
I hope this helps,
Lorraine