AITA for Exposing My Boyfriend at a Dinner Where Everyone Was Already Sitting?

I (27F) have been dating my boyfriend (29M) for three years and thought everything was normal until my best friend (26F) started acting weird around him at family dinners and my sister (24F) hinted she saw something at my workplace involving a coworker (31M) who also knows my boyfriend from the gym. At first I ignored it, but then I noticed late-night messages on his phone and he would step outside every time she called, which made me increasingly uncomfortable and confused about what was really going on behind my back. Last weekend I decided to test it by telling him I was working late, but I actually went to the restaurant where my coworker said he had been meeting someone, and I brought my best friend with me because I needed a witness in case I was wrong or paranoid. Inside the dim corner booth I froze when I saw my boyfriend sitting across from the coworker leaning in far too close, and my sister was already there looking shaken while my friend grabbed my arm telling me not to rush in he noticed.

For a few seconds I just stood there behind the divider, trying to process what I was seeing. My boyfriend (29M) looked up first, his face going pale immediately. The coworker (31M) pushed his chair back like he had been caught mid-crime. My sister (24F) wouldn’t even look at me, and my best friend (26F) whispered that I should stay calm, but I was already shaking.

I walked straight to the table and asked what exactly I was interrupting. My boyfriend tried to laugh it off, saying it was “just a work discussion,” but his phone lit up on the table with messages from the coworker that didn’t look professional at all. That’s when my sister finally spoke and said she had known for weeks but didn’t know how to tell me.

The restaurant went silent because apparently other people recognized them—my boyfriend works locally and so does the coworker. Someone at a nearby table started recording. My boyfriend panicked and tried to grab my arm, but I stepped back. My friend told him to stop touching me, and that’s when things escalated.

The coworker admitted they had been “emotionally involved,” whatever that meant, and that it had started months ago. My boyfriend kept insisting it wasn’t cheating, just “confusing feelings,” but nobody around us was buying it anymore. The manager even came over asking us to move outside because customers were uncomfortable.

After that night, everything collapsed fast. I ended things immediately. My sister apologized but said she was scared of getting involved earlier because she didn’t want to destroy the family. My boyfriend kept texting me for days, switching between begging and blaming me for “invading his privacy.” The coworker got transferred after HR got involved due to workplace misconduct complaints, especially since this wasn’t the first rumor about him.

Now my boyfriend is telling mutual friends that I humiliated him publicly and “set him up,” while others say I had every right to confront what I saw instead of staying silent.

So now I’m stuck wondering… AITA for exposing him like that in front of everyone instead of confronting him privately?