Having sex on ovulation day and taking Birth control pill within 15mins of sex ?

I make this lecture several times a day, so here is what I just said to a girl in another question.
kaismama

9 minutes ago

Calenders and apps can't tell you when you ovulate. . They are educated guesses. We don't ovulate according to an average and we don't do it the same time each month. The guess is you ovulate 14 days before your next period. Unless you're periods are exactly the same time apart each month, even the average would change the day
There is a chance of pregnancy because morning after pills aren't as effective as regular birth control. They have a 5-20% failure rate, that's the rate of possible pregnancy. the sooner you take it the better.
She can't keep taking the morning after pill. Either use a condom all the time or she needs to think about regular birth control.