Depends on the brand
Pretty much all oral contraceptives work by not allowing ovulation to occur. They contain low doses of estrogen and progesterin. This works by not allowing an LH and FSH surge right before ovulation were to occur. Since there is no surge, no ovulation takes place, and therefore no fertilization of egg and sperm can occur.
I think most of the pills say in the literature that you are safe after 7 days. I see no sense in waiting a month or more since she starts a new cycle ever 28 says and the pill doesn't build up in the body.