Agree with ppowb. Emergency contraceptive though is hardly worth the time if you aren't going to take it the right way. If she is late, take a pregnancy test. She likely isn't but stranger things have happened. The one pill may have created enough of a disturbance in her hormones to prevent conception from happening. Let's hope! Good luck
One pill could work but two would be better. Maybe she was not on her fertile days anyway - cycle days 8 through 16 of 28. She can find out more about that by downloading a period tracking app. If her next period shows up missing, that would be a good time to run a home pregnancy test. Best of luck!