S.J.
I use the velcro rollers and secure them with a bobby pin. I haven't used electric rollers since high school! If she is wanting softer curls and already has curly hair, I am not sure electric is the route she wants to go.
I roller brush my hair in sections, put the velcro roller in, then secure with bobby pin, and leave in as long as possible (usually an hour or so before I have to be ready - the longer, the better). Spray with some Paul Mitchell Finishing Spray - that is your best bet against humidity. No brand of rollers will be better than another at preventing a humid climate from ruining her hair. Those high humidity days may call for an up-do.