She saw a plane flying low and very fast. Nothing unusual in this area.
First, The Pentagon is less than a mile from Reagan National Airport. As stupid as it is some of the flight routes take planes very close to the Pentagon itself.
Second, they regularly have military fly-overs here at the Pentagon. Military aircrafts will fly over the building at low altitudes and at times at extremely high speeds.
Third, are you confident your friend can tell different plane types apart? Plane body styles are very similar to one another. There is no telling what type of plane she saw.
Again I can't say for certain what happened in Arlington, New York, or Shanksville. All I know is that anyone that dismisses the "other" theories of what happened are being foolish. You should at least consider other options. To accept the Gov't account of what happened as fact is a leap of faith I can't make.