Is plan b effective even if the girl just got off her period?

Usually. Conception can occur during any day of a womans cycle including menses. It is uncommon close to menses, but it does occur in some. Plan B creates a uterine environment that makes conception and implantation quite difficult.

Plan B is about 95% effective if taken within 72 hours of sex. It is advisable to use contraception every time, all the time. Get HPV vaccine. Wish you good health! Safe sex: https://www.hopkinsmedicine.org/health/wellness-and-prevention/safer-sex-guidelines