How does she know her ovulation- does she test for it or track symptoms? The female body can be kinda complicated when it comes to ovulation- and things can effect ovulation- causing it to be earlier or later than one may think or estimate. Also- sperm can live in the human body for days, so even if you didn't have sex right when ovulation occurred- pregnancy is still possible. It sounds like you are having unprotected sex- pregnancy is always a possibility any time you have vaginal intercourse (protected sex brings these odds down significantly, but never eliminates them)- so since it was unprotected and you did have sex- it's entirely possible pregnancy occurred.
If she has missed her period, she should go ahead and take a test to see what the results are.
Best of luck, and let us know if you have other questions and how the test turns out!