The following would be what I would actually answer to him, but after some research on my side, I didn't think that I would told the right answer, and there is some inaccuracy, can you help me to mend this? What I actually find: "baguette is what pasta for Italians, rice for Chinese, corn for Mexican, potato for Irish or beer for German, it's a daily consummer goods for French. Then for historical reasons, poor French peasants before industrial revolution need generally something that last for several days and have some pretty good nutrition value. So that gives birth of a 20 inches round bread that have the outside crusted and look like a 20 inches large bowl, poor French peasants pour vegetables soup inside of this kind of bowl (yes they even not have money to buy a bowl under French feudality most of times) to soften the bread. That kind of "tradition" last until Napoleon, where armies are unified with industrial equipment, as soldiers get metal bowl to get their soup, there is no more needing of bread bowl, so they need get some kind of bread that also last several days but in a long shape to match the idea of easy to transport by soldiers within their bags, so that gives baguette, nowadays baguette become way more soften that before as the need of transportation for several days and is no more a soldier's "equipment". But it last until now and won't be disappeare for a long time is that during the industrial revolution, French factory managers gives also this kind of food to workers. It names baguette only for its shape, there is many kind of French bread that is still calling "pain" (bread) by French." But the problem is that there is some document of the Paris metro that states baguette was invented by a engineer that is leading the constructing site of the metro to prevent workers bringing knife inside the site. Which one is more appropriate?
Hey there --- this turns out to be a tough question, and I'm sorry to say I don't have a definitive answer for you. But I did find a great resource by an American historian who specializes in French bread, Jim Chevallier.
He summarizes his research on the matter here, and it is extensive. But the short version is that while long, thin breads appeared in French bakeries as early as the 18th century, the exact form of bread commonly known as a baguette did not seem to be commonplace until the late 19th century, and the word "baguette" itself does not seem to be widely used to describe it until the 1920s. ("Flute" was a more common term for long thin bread earlier; its unclear exactly whether baguette and flute started out as synonyms for the same type of bread and if so when they began to differ.)
Chevallier does definitively debunk the Napoleon's soldiers and subway worker legends, but the change in the law banning night work for bakers (coupled with innovations in flour milling and yeast production introduced by Viennese bakers, along with steam ovens) might have helped play a role in popularizing the baguette, since the new techniques and the form of the bread enabled quicker rise and cooking times than the classic boule.