The typical length for the menstrual portion of the cycle is 3-5 days. This is the portion of the 28 day cycle in which bleeding
occurs. Thus, 2 weeks is a bit long for someone to be on their period. There can be several reasons for this. If this was a one time thing, then the cause will probably never be found. In some cases, a pregnancy that is lost very early on can result in this length of bleeding.
If this is a recurrent problem, then your girlfriend needs to have an evaluation for excessive bleeding during the menstruation. One common condition that can cause excessive bleeding on every period is a blood clotting disorder (called a coagulopathy). This is where the blood does not clot properly, prolonging the bleeding part of the cycle. Another possibly is that she has developed a benign tumor called a uterine fibroid. These tumors do not progress to cancer, but they can be a cause of excessive bleeding.
I suggest that your girlfriend schedule an appointment with her OBGYN. He or she can take a more detailed history of her symptoms and perform a thorough pelvic exam. She will likely want to get coagulation studies to look for a blood clotting disorder. In addition, she may want to get an ultrasound
of her pelvis to look for any abnormalities in the uterus. I hope she gets this figured out.