Will i know if im pregnant if i get my period or not right after i have sex?

my friend had sex the day before and the day she got her period..like she got her period later at the end of that day, and he came on her close to that area but she doesnt think any *** went in but isnt positive. if she was pregnant, would she not have gotten her period when she was suppose to later that day, ? or would she still get her period if she was pregnant just because her period is so close to the time she got impregnated?