It doesn't, thankfully, happen in the Great Hall. They're in a side hallway after dinner, and Remus is sure they've made it through the day without incident.
Which is when, typically, Regulus shows up and calls Sirius an embarrassment, a blood traitor, and a waste of a Black.
Sirius, relaxed and laughing moments beforehand, instantly stiffens.
"If you're going to insult me, at least try harder," he says, and Remus is already imagining how many detentions the four of them are going to have later.
Regulus sneers. "Oh, that I can do, mudblood-loving bastard."
Remus visits Sirius in the hospital wing later. He doesn't bother to say I told you so.