Ovulation (which lasts about 48hrs) generally occurs about 2wks after the beginning of a period and sperm can live inside the reproductive tract for up to a week. Given that these are average numbers (as in ovulation can occur earlier you don’t really know unless taking an ovulation test and that’s still an iffy window) and not knowing how long her period was, there’s actually a fairly decent chance she could get pregnant if she doesn’t take Plan B. Also, the efficacy of Plan B drops quite a bit with time (95% w/in 24hrs to ~60% w/in 48-72hrs, most effective w/in 12hrs after sex) and is also affected by things like BMI and drug interactions so it’s not foolproof.