Your girlsfriend is perfectly protected during her placebo week!
As long as she takes her next pack on the 7th day like she is supposed to all is well..
But since you seem like a proactive young man, and you want to know things.. I have some advice for you...
Have "the talk" about what to do if she gets pregnant on birth control(highly highly unlikely) ..Will she have an abortion? Will she keep it, adoption? You will feel better, and you will probably feel more at ease if you just know what happen "if" pregnancy happens.
But still, it is hihgly unlikely, and she is well protected on birth control.