As to how it works....
The emergency contraceptive/morning-after pill has three possible ways in which it can work (as does the regular birth control pill):
Ovulation is inhibited, meaning the egg will not be released;
The normal menstrual cycle is altered, delaying ovulation;
It can irritate the lining of the uterus so that if the first and second actions fail, and the woman does become pregnant, the human being created will die before he or she can actually attach to the lining of the uterus.
In other words, if the third action occurs, her body rejects the living human embryo, and the child will die. This result is a chemical abortion. (Abortion is an act of direct killing that takes the life of a living preborn human being�a life that begins at fertilization.)
So it may be that her taking the pill has delayed her period anyway.
As to finding out for certain if you're firing blanks, see your GP. If he or she is concerned, they can probably arrange a test.