My best guess would be that she is probably not pregnant. The chances are VERY slim, because if she has a normal cycle, she would not be ovulating for about 7-8 days after you had sex. I believe sperm only survive for about 72 hours to five days. But there is always a "chance". As for the waiting period, just try not to think about it too much, because like I mentioned, stress can delay your period, and that makes the situation worse. Hope everything works out for you!
P.S. If she is really that nervous about it, you can buy a good pregnancy test that can detect the pregnancy hormone up to 5 days before you even miss you period. But, these don't always work, and if it comes out negative, they always say to test again if you miss your period. I used one three days before I missed my period, and it came up positive.