Other than Plan B u cannot stop it!!!! Oh unless U don't *** inside next time use a condom!!!!
She can take it up to 72 hours after the unprotected intercourse. However, definitely have her take it sooner rather than later. As ItstheSky pointed out, it becomes less effective as time goes by. This is because conception usually doesn't take all that long to occur if the woman's already ovulated.
Definitely use protection from here on out because it's bad to take Plan B all too often. Note that once she takes this, she could have an earlier or later period than usual and it can be a bit different than usual due to the high dose of hormones she will be taking.
There's a pill called "Plan B" also known as the "morning after pill". You have to go to your local pharmacy ASAP and ask them for it. They keep it locked up, because I think you need to be of a certain age to buy it. I think 18...but not sure. She needs to take it IMMEDIATELY the morning after because as time goes by it becomes less effective. It's not 100% effective but it usually does work.
I recommend using other protection next time if pregnancy is unwanted or there is a fear of it. Abuse of the morning after pill could cause major hormone issues, acne, mood issues, and infertility. Condoms and normal, daily doses of birth control are the best way to go.