There’s several issues here. One is the timing, which is discussed in the linked article above. However, the article calculates the odds simply on a random basis. They use the figure that 3 days out of a 28-day cycle in which sex can result in a pregnancy for 11%, and then say that pregnancy only happens 1/4 to 1/2 of the times when there are sperm and an egg, for a result of 3 to 6%.
However, if the friend is worried about pregnancy, we can assume that it was closer to her three-day window, and the odds are higher than simply random.
The second issue, though, provides another reason why the odds quoted above are meaningless. Unlike blackjack, where the odds don’t care who has bet their ranch that the dealer will bust, for pregnancy, the chances are highly dependant on the players involved. Is she young? Old? Does she have any fertility issues? Does he? Does he shoot blanks? Is there a high enough concentration of sperm?
Ask the couple who are undertaking fertility treatments, faithfully measuring temperatures and repeatedly failing for years, and balance that with the women who do get pregnant in one shot.
A lot if known about what problems cause infertility, but medical science can’t say why some people are super fertile.
My mother had five pregnancies (including the first which ended in a miscarriage) in a little over four and a half years, in spite of increased use of birth control and breast feeding.
My wife has gotten pregnant three Januaries in a row. One of her doctors, a top fertility specialist says that this isn’t unusual. Often women can only get pregnant a few times during the year (and obviously not the same year).
Other times it just doesn’t happen. (And here’s my wife’s pet peeve.) Despite the evidence that half of infertility issues are from the man, the blame often is more likely to be placed on the woman.)
So the short answer. No one knows for any particular person.