This is very common and mostly nothing.
Obviously, if you are having unprotected sex, there is the possibility that there was a pregnancy.
However, long heavy bleeding is not trivial if it is really heavy (at some point anemia is a factor) but nobody can tell if it is that kind of heavy.
Most urgent care places in most civilized countries (sorry if you are in texas or Florida) have all that is needed to do a quick check for the most common issues – a blood test can ensure she is not anemic or near that, a pelvic exam and ultrasound and pregnancy test can rule out most of the other things.
She should make an appointment with her OBgyn and make sure she has regular yearly exams.