Of course! It sure helps to use a condom as well, or a different form. But the pill is still 99% percent effective whether or not you use another form.
The pill supposedly has a 98% effectiveness rate. It does nothing to protect you from STD's.
Remember that birth control pills aren't foolproof. Using other medications, skipping pills, illness can cause the pill to become less effective. There have even been a few rare instances of women becoming pregnant while taking the pill.
If your girlfriend has been taking the pill every day without fail and at or around the same time, her chances of getting pregnant are very slim. I have heard that with perfect use, 3 in 1000 women per year can get pregnant.
I just found out Im pregnant and I conceived on the pill. I take my pills every morning at 8:05 when i brush my teeth. I am not sick or on any other medications. Turns out some food, too much dairy, or too much deer meat or anything can throw the pill off.