but according to the article linked below, he would have received only a slap on the wrist:

...duelling was illegal and the participants could be arrested merely for attempting to fight a duel. In practice few people suffered serious legal penalties for duelling; as long as the rules of honour had been followed, those duellists who killed their antagonists and were prosecuted for murder were invariably pardoned or convicted of manslaughter and given token punishments.

Interesting, and highly pertinent to our novel, is the fact that this was because of an increasing "sensibility" among the English population generally. (This led, among other things, to the abandonment of public hangings at Tyburn.) Prospective duellists were likely to be stopped by the public, or even by their own seconds.

...overall levels of public violence by gentlemen were declining dramatically in London. Violent behaviour was increasingly condemned in urban gentry culture. Gentlemen became subject to the ideals of politeness, in which men were expected to control their emotions and be generous and complaisant towards those with whom they interacted. From the
1760s and 70s, the ideals of sensibility required men to show even greater sensitivity and sympathy to other people's feelings.

Duelling was, in short, generally unpopular except in specific sectors of the population. Given this, it seems as if the "sensible" Willoughby as well as the colonel were flouting popular opinion when they made this "appointment" - which would have been in a remote place at a time (e.g. dawn) when not many people were about, in order to avoid interruption.

As other posters to this thread have pointed out, the only one who really knows what went on that day is JA, and she ain't talkin'. However, I do find it intriguing to speculate on why he called Willoughby out in the first place. What good would that have done? Katherine W suggests that it was to avenge both his and Eliza's honour. But what if he had been killed? Could Eliza have been guaranteed protection then?

BTW, Jeffrey - you're right that it wasn't fisticuffs. They were reserved for the lower orders. The whole idea of duelling, especially with pistols, was to lower the emotional temperature of the affair.