She's right. The luteal phase (time between O day and period) may only vary by 1 day on either side each month...so if it's usually say, 14 days, you can expect AF 13-15 days after O.
most of the information i have read says the time from ovulation to your period is usually about 14 days give or take a couple and that the time between the first day of your period to ovulation is what differs widely. so the time between ovualtion and your period should stay close to the same every time you have your cycle
hope that helps