Does anybody actually have control over the narrative
To a degree. It's circumstantial and I suppose it depends on what lengths you are willing to go to.
In my own situation, given that the affair was between my partner and my best friend, they immediately launched a full-blown smear campaign against me. They denied everything to anyone who would listen, actively trying to convince our inner circle that I had completely lost my mind and was just seeing ghosts. I didn't have absolute, ironclad proof at first—no intimate photos or full transcripts—just a suspicious taxi receipt and screenshots of deleted middle-of-the-night text messages. It was enough to know I wasn't crazy, but hardly a smoking gun.
Determined not to be painted as the paranoid ex, I made it my personal mission to show every shred of evidence I had to anyone willing to listen, eventually winning people over. The absolute nail in their coffin came when one of their few remaining loyal defenders caught them kissing in public. Backed into a corner, they swore blind that they had only started dating after the breakup, driven together by my groundless accusations. Naturally, even their last handful of believers saw straight through that fairytale. I still fondly recall the excruciating pub meetup where the poor, deeply awkward guy who caught them practically begged for my forgiveness for doubting me.
Ultimately, I invested heavily in controlling the narrative, and it paid off handsomely. By the end of it, they hadn't managed to keep a single friend from our massive shared circle, eventually exiling themselves to a miserable part of town, never to be seen again.
Which brings me back to the main point: if you actually have total, undeniable proof—full screenshot receipts, videos, or a written confession—you have all the leverage. You don't just have to sit back and take the social fallout; the narrative can absolutely be controlled. If I had the transcripts Id have publicly spammed everything that I was within my legal right to.