Did she use first morning urine? If so, if it's been a month since your encounter, and she's still not testing positive, it's unlikely she's pregnant. Stress can sometimes delay her menstruation.
UPDATE: she still haven't got her period and she also said her waist started to cramp the other day and now her stomach hurts pls help:((
Yes, using first morning urine is important. The pregnancy hormone is more concentrated. I'm not sure what you mean by "her waist hurts" but some women experience cramping in early pregnancy. Cramping is a more common symptom of a period about to begin, though. Common symptoms of early pregnancy include very tender breasts, (which are also a symptom of a period about to start, but in early pregnancy the tenderness is more noticeable) and nausea. Can she take another pregnancy test, this time with first morning urine?