Really depends on her cycle, is she regular? Does she have normal menstruations? Chances are slim - however everything is possible if she's not on the pill & you're not wearing a condom
Here's something from 4women.gov:
Your date of ovulation is calculated by subtracting 14 to 16 days from the date of your next period. This is an approximation and may not be exact for every woman. These calculations may not be accurate if your cycle is less than 21 days long or more than 35 days long, or if your menstrual cycle is irregular from month to month.
EDIT::
To add to the 'old wive's tale' - was she referring to this?
You are most likely to become pregnant if sexual activity occurs right before or after ovulation.