Yes, it will prevent pregnancy. The pill is 99% effective.

hi, but i heard the pill is only effective after 7 days? but she consume only the second day, will it still be effective?

If the pill is taken continuously, every day of the month, then it will be 99% effective every day of the month no matter what. This means that you are protected every day of your placebo week, and every day when starting a new pack.
The pill is only effective after 7 days when you FIRST START TAKING IT. But, since your girlfriend had been on the pill 3 months prior, she has been protected (as long as she's been taking it) every day since the first 7 days she began taking it.

meandyou, the pill becomes effective after the first seven days, the first time she takes it. As rockon0159 said, she has been protected every day after the first seven days.

hihi, thanks you! i get you

If you read the prescribing information/drug fact sheet that comes with the pills it tells you to use additional protection during certain times....like the week after you start your period. Look up the fact sheet and see for sure. Also, if she doesn't take the pill at the same time everyday then it won't be as effective. My mother was on the pill but didn't take it at the same time everyday so they had me! It's not 100%. If you don't want a child then be double protected at the very least - that or don't have sex! Good luck.
BTW - if she's really worried and thinks she's pregnant she can take the plan b pill within 72 hours of having sex. Not as effective the later it gets though. But know that the plan b can cause some bad side effects like severe nausea, vomiting, stomach pain, fatigue, and other stuff that's no fun at all.

My advice to you and all guys out there is to wear protection whether your girl is on the pill or not because you don't know if she's taking it right or if she might have missed a pill or whatever. Better safe than sorry later on.

Good luck.