Although birth control pills are totally effective but still a lady can get pregnant while taking it.
This happens not because that pill has failed to work but because of other reasons like
Forgetting to take the pill
Not taking the pill daily on same time,which is very essential
Taking certain medications along with the birth control pills like antibiotics,anti seizure
medicines,anti fungal medicines etc.These medicines reduce the effectiveness of birth control pills.
So essentially your girl friend has very remote chances of getting pregnant but she can still get pregnant.