Pablo Picasso once said "Art is the lie that reveals the truth." Except that he didn't really say that. What he said was "Art is a lie that makes us realize truth, at least the truth that is given us to understand." (The Arts: An Illustrated Monthly Magazine Covering All Phases of Ancient and Modern Art, NYC, 1923)
Is there a subtle truth to your friend's lie? It need not be related to scallions. We perceive the world through narrative. Perhaps your friend was introducing a fundamental truth through the revelation of scallions growing in a bathroom. Or at least that's how we would interpret it on my home planet of Zeta Reticulii IV. Of course the "fact" that I'm from Zeta Reticulii IV is a lie. I grew up in Texas. You can make your own decisions regarding the relative adherence to consensual reality between Texas and Zeta Reticulii IV.
Perhaps the story of growing scallions in the bathroom is nothing other than the creation of a shared history. Does it matter that history is counter-factual? We're social beings. We do things like that.