It's choose your own adventure time! Would you like to see...
A) Devon and Melanie have a fight.
B) Christo walk comedically into the door.
C) Peter kick Devon's ass like it's never been kicked.
D) The Kool-Aid mascot bust through the wall and offer refreshments.
Option C must not come before A. Or Melanie's definitely lost to Peter (as a friend I mean) and the gang, so she won't ever come back into the story except maybe as a cameo in Snapshots.
So yeah, B. A. C. Christo goes off funnily, Melanie can't help laughing and Devon gets pissed off, he yells, Christo and Mulligan stoke the embers, Melanie finally yells too, Devon slaps her - and boom.
But if I get to choose, how about this variation? Christo makes Melanie laugh as above, Devon yells at her as above and Peter looms in on the fight with an uncharacteristic air of quiet menace. (A skill he learned in his new job.) He waits for Devon to catch his breath, and then asks him real nice like to apologize. (As real nice like as can be done with the hint of a savage growl in undertones.)
From there on, Devon yells some more, catches the glint in Peter's eye and cowers. Badly. Like a rat before a lion in the seconds before it realizes it's been cornered. Melanie sees a bit of what Devon truly is, and the first little crack in her slavery appears.
Of course, I'm asking a lot from Peter as I don't see him maturing enough to make this work. A better idea would be to use teamwork: as above except Mulligan does the talking (including asking Devon to apologize) and Peter growls softly behind him to drive the point home.