This is a great statement to discuss with regard to this play and these two characters. To be honest, I can see both sides of this issue. For example, on the one hand, a basic comparison of these two characters reveals that Macduff was assertive and left Scotland deliberately to join the forces of Malcolm against Macbeth. Banquo, however, in spite of his considerable grounds for suspecting that Macbeth was guilty of Duncan's murder, did nothing to oppose or challenge him. Note what Banquo says in his soliloquy at the beginning of Act III scene 1:
Thou hast it now--King, Cawdor, Glamis, all
As the Weird Women promised, and I fear
Thou played'st most foully for 't.
In spite of his clear suspicions of Macbeth's involvement in the crime of regicide, Banquo does nothing to challenge Macbeth. We could therefore argue that Banquo missed an opportunity to challenge Macbeth and perhaps stop him before he committed even greater crimes. Macduff, on the other hand, moved to oppose him and successfully did something to stop Macbeth.
However, I also think that we can be a bit too harsh on Banquo. Let us remember that Macduff moved to oppose Macbeth after Macbeth had killed Duncan and killed Banquo. Macduff had longer then to see evidence of Macbeth's villainy. Banquo only saw evidence of Macbeth's involvement in the killing of Duncan, and we can imagine that he was rather bewildered at what happened and how it followed so quickly after receiving the prophecies from the witches.
Therefore, on the one hand, the character of Banquo could be used to suggest that good men doing nothing could result in the rise of evil, and Macduff shows the opposite of this statement. However, let us also remember the course of events and the way that Macduff had more time to see greater evidence of Macbeth's treachery.