I think the Colonel would look to Elinor and act accordingly. If Elinor seemed to wish to talk to Willoughby, I have little doubt that the Colonel, Ultimate Gentleman, would withdraw and let them talk. However, if Elinor seemed to need rescuing, I also have little doubt that the Colonel would escort Willoughby outside (if Willoughby hadn't already exited). I do doubt that he would challenge him to a duel, at least in front of the Dashwood ladies.

Perhaps that is a L&T question ~ would it be out of place to challenge someone to a duel in front of a lady? It seems to me that it would be, but then, I suppose gentlemen at that point aren't thinking of manners! ;-)