Is my friend Pregnant or Not?

A friend of mine came to me today totally freaked out becasue she and the guy she has been seeing think she is pregnant. According to her, they have a had a few times, and they used protection with all but the last time, which was only about 5 minutes. Now, she is worried becasue she hasnt had her period in awhile and she says its been a little over a month starting next week. Now being a guy I have no idea how it really works, but is it common for women to not have there periods over long time intervals but are not pregnant or should she be worried.