Birth control

Huge question. Someone I know had unprotected sex and the guy said you might want to get plan b. She doesn't plan on taking plan b but said she is going to get an IUD in a week. If the sperm reaches the egg before she gets the IUD, would the IUD stop anything from occurring? I told her she should get plan b because a week is a long time but she doesn't think she needs it. Ladies, I need your advice.