They are affective while she is aroused
Ordinary birth control pills don't work like that. You have to take them for a while and regularly all the time as prescribed.
Because you have had sex today, the type of pill your partner needs is the Emergency Contraceptive Pill, also called The Morning After Pill. This HAS to be taken within 72 hours of having sex for it to work to guard against any unwanted pregnancy.
Your pharmacist should be able to assist you if you can't see on the shelves in the pharmacy shop. The doctor could give a prescription.
Do remember it MUST be taken within 72 hours of having sex.
In the future, do not rely on the Morning After Pill, it can mess things up for the woman. Best to use latex free condoms.