I would not rule out Coccidiosis. It can kill large numbers of birds very suddenly with no clinical signs. This is the right age, too. The only way to know is to do a necropsy and look for thickened mottled intestines, particularly in the proximal small intestine, ceca and colon. A fecal exam can also detect them. It is contagious but most commonly kills the younger birds. Older bird (I believe) build up an immunity, but may succumb if they are stressed. It can't hurt to be too careful and practice good biosecurity, ie limit contact between her birds and yours as much as possible, including tracking stuff on your shoes if you go over there.