Birth control pill after ?

She should take Plan B (the "morning after pill"), and she needs to take it asap. Birth control pills are only effective if you take them everyday as directed and only after you've been on them for a full month. They aren't magic, they are a daily horomone that prevents you from releasing an egg, so no, just taking one will not do anything at all unless she's already on the pill. Plan B is intended for just this situation however.