She shouldn't be pregnant if that's what happened.
if she is worried, take a test and if its positive...well, like the pp said, you'll be needing a long chat.
also I strongly advise her to learn about her cycles more; as in when ovulation occurs, fertile window, changes in cervix etc. this knowledge will help so much both for when not wanting to conceive and later if she wants to. until she decides its time for a baby however, hormonal contraception as well as a prophylactic is highly recommended.
If it was her first time having sex overall, or in a while, then that may be why she's late, sometimes sex can push a girl's period back, it can also be stress, such as the one caused by not knowing whether she's pregnant or not, I'd say she should take a test to be sure, the chance of her being pregnant, by what you descried, should be none, if in any case, she is, then you two may have some talking to do...