question for my little sister--birth control

Hey my little sister came to me and told me that she came off of her birth control for a month because she messed up and the pharmacist told her to wait for a new pack. It wasn't a whole month but it was a good 3 weeks. She came to me and told me she had unprotected sex with her boyfriend on Christmas and the day after. She also said that Christmas was the day she started her new birth control pack after being off it for a while. She's worried. She's very cautious but is wondering if she should get a plan b. Figured I'd ask on here cause who knows better than a bunch of women ttc like me. Is she covered under birth control or do I need to run out and get her a plan b?