Advisor drops MSci student suddenly in final semester
Advise your colleague to take this up with whatever graduate student support is available at your institution.

As a fellow student, there probably isn't a productive way for you to intervene directly.  You don't have any official role or reason for involvement, and any action you can take could be taken more effectively by the student herself.

The situation you describe is quite troubling.  If this is the whole story, then action ought to be taken (and hopefully the university will support the student).  But she really needs to be her own advocate here.

However, keep in mind that you only know one side of the story.  Even though you think quite highly of this student, you need to be cautious in jumping to conclusions.  Sometimes (though not always) when something seems inexplicable, it's because there is more to it than meets the eye.  So I would be careful not to take any rash actions.  Don't let this unduly affect your professional relationship with your adviser.