Is this copyright infringement and if so, should I do anything?

My friend and I are both art students. She recently sold a drawing for a sizable amount. By coincidence, I was flipping through a figure drawing book today and saw the exact.same.drawing. Except for it was by a professional artist – not a famous one, but well-known enough to get into a book, I suppose. She had copied the drawing.

Is this copyright infringement or just unethical? Is there anything I should do? Would confronting her even be my place or should I just let it go?