Accidental impregnation

So a friend of mine had sex and her boyfriend's condom broke so he accidentally came inside of her, and that same day she took a pill (a contraception pill or morning after pill, something like that) and she scared because she hasn't had her period yet, I keep telling her that the pill will cause you to have an irregular menstral cycle, she kind of has a hard time believing it. I need something solid for her to believe that won't make her doubt into thinking she's pregnant.