If you take the plan b pill what happens?

i just wanted to know what happens when you take the plan B pill ik its supposed to stop you from getting pregnant but my friend was having with her boyfriend and the condom broke and like they didnt know..but she has him pull out whenever hes about to come. but shes worried about the pre-*** factor and just wanted to know even though she took the pill an hour later what are the chances of her becoming pregnant? ohh and also she got a spotty dark period about 6days later but i told her that comes with taking the plan b pill..was i wrong?? please help! thanks!