birth control pills (combined prog+estrogen) are over 99% effective if she follows the instructions perfectly (around 2 hour window every day), although its only considered a missed pill if its taken over 6 hours after its supposed to be taken
she can take 3 month packages, that way she only gets her period every 3 months, and youre protected all year round because the estrogen levels dont drop low enough for a normal ovulation (do some research though, im saying this based on the seasonale pill, others might be different)