My friend and her boyfriend had sex while she was on her period she takes birth control on the days she supposta to, should she be worried she's prego?

Probably not!: The least likely point in the cycle that she might conceive is during her period. Since you are asking for your friend, i will assume that you might not know about all of their activity. Your advice to her would be to check a pregnancy test now and again in about 2-3 weeks. She should also consider some form of birth control or have condoms readily available. They are good for preventing stds!