Pregnancy is dated from the first day of her last period. So she could be 5 weeks along. And, I'm sorry to say this, but if your friend was so concerned about it, he'd have worn a condom.

If her period was due on May 15 and she has a 'classic' 28 day cycle, meaning the first day of her last period was April 17th, she could have gotten pregnant on May 5th. You start the countdown of how many weeks you are from the April 17th date. So right now she would be 8 weeks 4 days pregnant.