Menstruation or 'periods' is the time when the unovulated egg gets torn from the wall of placenta, and comes out through vagina.. So till the date of the 'period' the egg is inside the placenta and if an intercourse is made before the 'period' there is high possibility of ovulation.. And an intercourse just after the period - within seven days from period - its very very low chance of ovulation as it takes atleast seven days for the new egg to travel to placenta.. so in your friends case, the intercourse before the 'period' should have been the cause for pregnancy and not the one after 'period'.. and remember that the sperm can live maximum of five days inside a woman's body..