It sounds like your friend is relating a story associated with the social interactions between Queen Louise and Napoleon during the negotiations for the Treaty of Tilsit. The Prussian Queen attended Tilsit with her husband, where the French ignominiously quartered them in a flour mill. Louise was very much involved in trying to charm or cajole Napoleon into granting Prussia a more lenient peace.

The type of story the OP's friend relates fit very much in the mold of the mytholgization of Louise throughout the nineteenth century. This work, which ranges from popular novels like Luise Mühlbach of the near hagiography of historians such as Johann Gustav Droysen, portrayed Louise as willing to sacrifice anything for Prussia, including using her feminine charms on the Emperor. These portrayals of the royal couple often stressed their love match and deep seated love for each other and for Prussia. These portrayals also emphasized Louise's great beauty and intelligence. King Frederick William III bursting in on his wife swallowing her pride and seducing the Emperor for Prussia fits in this mode of depicting these Hohenzollerns as equal parts patriotic and domestic partners.

All of this mythology around the royal couple makes finding out the actual events at Tilsit much more difficult. Napoleon's surviving correspondence does indicate that he felt that Louise was trying to seduce him, but there was little to indicate she was successful. Napoleon reportedly called Louise "the only real man in Prussia," but that was more illuminating of his attitudes towards the often quite passive Frederick William III than Louise. She was quite strident in wanting Prussia to retain Magdeburg and some of the fortresses on the Elbe, but Napoleon steadfastly refused. One story from Tilsit was that after Emperor Alexander I and Frederick William III retired from a dinner, either Napoleon or the Queen offered a rose (who gave the rose differs in accounts), to which Louise asked again for Magdeburg "for her children," along with the rose. Napoleon refused and variations of the story have Napoleon saying he wanted the rose, not Louise. Napoleon's letters to Josephine do contain a degree of scorn for Louise trying to think she could seduce Napoleon out of victory. Naturally, such dismissals need to be taken with a grain of salt since Napoleon was always proactive in trying to dismiss rumors of infidelity, but this derision does fit into his preferences in women. Napoleon generally preferred more passive and refined women like Marie Walewska or Marie Louise. His interactions with the formidable women in Parisian salon culture like de Staël were often quite negative. Louise herself was also not quite the beauty in Napoleon's eyes that subsequent mythology would have it with the French Emperor merely commenting she looked good for her age of thirty-five.

As for the OP's direct question, I did not find any substantiation of the story. It sounds the OP's friend was conflating the tale of the dinner rose and the wider romanticization of Louise and her husband as a pure love-match. The latter was so common in the nineteenth century that they have filtered into public memory through various stories and anecdotes that illustrate such a deep domestic connection.

Sources

Hagemann, Karen. Revisiting Prussia's Wars against Napoleon: History, Culture, and Memory. Cambridge: Cambridge University Press, 2015.

Maierhofer, Waltraud, Gertrud M. Roesch, and Caroline Bland. Women against Napoleon: Historical and Fictional Responses to His Rise and Legacy. Frankfurt: Campus Verlag, 2007.

Schulte, Regina. "The Queen–A Middle–Class Tragedy: The Writing of History and the Creation of Myths in Nineteenth–Century France and Germany." Gender & History 14, no. 2 (2002): 266-293.