They are suffering from Phytophthora fungus. It is a very common problem with Catharanthus roseus particularly in hot, humid, rainy climates. She needs to find a resistant variety, like the 'Cora' series, or try another type of plant entirely. The spores are in her yard and will re-infect the new Catharanthus roseus plants. I've given up ever buying the generic 6-packs of these because of this very reason. In fact, I often see shelves of the cheap Catharanthus roseus 6-packs at the big box stores that are already infected and wilting.