I have a friend who is worried about possible pregnancy.

Her period is about 4 weeks late now and is now suggesting that her breasts are bigger.
The only incident that could have gotten her pregnant occurred about 2.5 weeks before her missed period (about 6.5 weeks ago).
It was not sex. Sperm ended up possibly on or near the vagina. She is not on birth control.
She has taken a total of 4 store bought pregnancy test: 3 days after missed period, 4 days after, 3.5 weeks after, and 3.5 weeks after.
All of them are negative.

Should she be concerned enough to visit a doctor?