Yes she can.
Each ovary is active every second month, if there is any irregularity, another ovary can reliease egg in the time when she still has a period, or day after: BTW, sperm lives 72 hours (4 days) in uterus, so...
That is the reason protection (condoms) is needed always.
How can you know?
She needs to do pregnancy test in 2-3 weeks of time and then repeat it week after. Hopefully everything is ok.