If she is concerned she might be pregnant, she can go to a local health clinic and get tested for free! There is always a chance that she could be pregnant, but I dont think its very likely. All of my friends, including myself, had a very positive pregnancy test when we found out that we were going to be parents. But everyone is different and every pregnancy test is differnet. My best suggestion is she can wait and take another in a week or so, or just go to the clinic.
Hope this helps!!!