Your friend was almost certainly referring to the Vendemiare Uprising, in which Napoleon defended France's new Directory government from a royalist mob.

While much of the Revolution took place in Paris, the outlying provinces were often much less enthusiastic. Most famous among them was the Vendee, situated in western France near the mouth of the Loire river. Here, Republican armies were locked in a savage guerrilla war with the royalists, who had British military and materiel support. While they suffered many defeats, the embers kept smoldering, and the landing of the Comte d'Artois (future king Charles X) further inflamed the situation; Royalist armies began marching on Paris in October 1795. Even though Paris was the epicenter of the Revolution, there were still many Parisians were sympathetic to the royalist cause, depending on which city district.

Moreover, the Parisians were incensed by their government's political dealings. Acting pretty blatantly to preserve their own power, the National Convention ruled that 2/3 of them would retain their seats of power after adopting the Constitution of Year III, which established the Directory. Tensions boiled over the the Le Peletiere section, and to make matters worse, thousands of National Guard defected. General Menou was dispatched to disarm the rebels, but his repulse only invigorated them. Sensing weakness, they marched on the Tuilieries palace where the Convention was meeting. Characterizing these people as civilians is complicated; many of them were armed National Guard, and in any case even unarmed civilians in large enough numbers had already proven very dangerous earlier in the Revolution.

Paul Barras, who rose to prominence in the Thermidorian Reaction against the radical Terror government, was appointed commander of the Army of the Interior and charged with protecting the government against the people. However, Barras was no soldier, so practical direction of the 5,000 regulars at their disposal was vested in a young but promising artillery officer, Napoleon Bonaparte. Napoleon later said that he was approached with this offer while at the opera, and that he spent a while soul searching before accepting, but this is almost certainly a distortion; he more likely bargained with Barras over a quid pro quo.

Napoleon dispatched a cavalry officer named Joachim Murat, his future brother in law, to fetch a park of 40 artillery pieces with which to defend the Tuileries. With adequate firepower secured, Napoleon carefully positioned his guns to cover the approaches to the Convention, and had a horse shot from under him in the fighting. He probably never said anything about a 'whiff of grapeshot', this being an attribution of the British historian Carlyle, but the royalists got more than a whiff, and with hundreds dead in a cannonade lasting hours, finally dispersed.

For his service, Napoleon was promoted to General of Division, the highest military rank in the French Republic; he was made second in command, then general in chief of the Army of the Interior. Derisively nicknamed 'General Vendemiare' for the circumstances of his promotion, was then reassigned to Italy, where he really made his name as a commander. For what it's worth, Napoleon never seemed to regret his role, and called 'General Vendemiare' his 'first title of glory'.

More broadly, the events of the uprising symbolized two crucial linked developments in the course of the French Revolution. First, the Paris mob became less powerful; they could no longer storm palaces and depose governments in fits of rage. The support of the National Guard, representing as they did the people in arms, had once sufficed to make and break revolutions, but that was no more. As a consequence, the government became profoundly military in character; the Directory was saved by the Regular Army at its very birth, and in a few short years, the military would overthrow the Directory it saved in Napoleon's coup of Brumaire. The army would finally pay back Napoleon in his own coin; once Paris had fallen to the Allies in 1814, the marshals Napoleon had raised to preeminence in his empire refused to follow him. Napoleon abdicated and went into exile.

Saying he fired on "civilians" is strictly true, but also a bit misleading, given their intentions.

Napoleon was already a minor hero in republican France for his actions at the Siege of Toulon in December 1789. As the commander of the city's artillery, he'd lead a daring night assault to overrun several British artillery batteries. Napoleon had been wounded in the battle, giving him further war hero cred.

The "whiff of grapeshot" incident you mentioned took place in Paris on 13 Vendémiaire (5 October 1795 in the French revolutionary calender). An army of pro-monarchist, Pro-Catholic French émigrés had landed in France with British support and was marching on Paris. The advancing army excited mass uprisings in Paris.

After the toppling of Robespierre and the Jacobins in 1794, France had seen street fighting between the jeunesse dorée, also known as the muscadins (middle- and lower middle-class dandies) and the radical sans culottes. 1794 and 1795 had been marked by several riots, street battles, abortive uprisings, lynchings, executions, show trials, and the anti-Jacobin White Terror.

With the prospect of a royalist takeover of the city, the jeunesse dorée, Catholics, National Guard defectors, and others flooded into the street. The ruling National Convention realized the seriousness of the situation and jumped at then-brigadier general Napleon Bonparte's offer to help, provided he was given free rein to do as he saw fit.

Napoleon sent Joachim Murat (who would later become a Marshal of France) and some cavalry to gather artillery from a Parisian artillery park. Murat returned with around 40 guns, which Napoleon sited around the National Convention's chambers to block the royalists' advance into the center of Paris. As an artilleryman, Napoleon knew guns could be an equalizer against the tens of thousands of Parisians moving against the Convention.

At his orders, Bonaparte's gunners fired grape shot (or possible canister shot) into packed, partially-armed crowd for about 45 minutes. About 300 bodies were left on the street and steps outside the Saint Roch church.

The event made Napoleon a hero to the Convention and its supporters. Napoleon was promoted and given command of the revolutionary French troops campaigning in Italy. Napoleon's successes in these campaigns made him a military superstar in France

The incident was somewhat flippantly summarized by Scottish historian Thomas Carlyle as a "whiff of grapeshot."