When I worked night shift, I had several occasions where I came close to having an accident and finally did have one. I managed to have a concussion, whiplash and bruises all over which sure beat the alternative.
Working 12 hour shifts, and depending on how far she had to commute could be a big factor. I have friends who work 12 hours and have an hour long commute home, have families. Talk about chronic sleep deprivation! Also, the hospital could have been using the usual "guilt" crap to get her to come in, or did they have a required call in/overtime policy? Many questions.