What are the chances?

We had unprotected sex (stupid I know), she uses a period tracking app where he had her period from the 20th of January to the 24th, before that being the 22nd of December to the 26th of December. We had sex yesterday where it saids she was ovulating 4 days ago. Her periods are usually pretty but not perfectly regular, she’s used the app consistently for a good few months.

Would plan B or EllaOne be affective in this stage? If so I will get her one ASAP