Something like that was handled very well I think. Bottom line was not to leave her waiting for the bus alone without an overwatch.
Good instincts! Could have been nothing to worry about... but could have been tomorrows next headline on CNN!
One question: Was the girl oblivious to the guy or was she sufficiently concerned about the situation?
Good job at paying attention to what was going on around you! Lots of people feel safe in a car and therefore tend not to just casually scan the outside world for anything thing untoward happening to someone else in the area.