The story, which made its way to r/JusticePorn (which is, unlike the name suggests, SFW - it's just a collection of stories about “justice being served to wrongdoers, miscreants and ruffians”), reveals that the guy was at a Lions-Bear football game when he spotted a couple in front of him.
"I said 'I don't know you and you don't know me, read this when you get home, TRUST ME.'"
The woman, who was pregnant, was repeatedly texting another man (“Jason”) throughout the game.
According to our Good Samaritan, she was writing stuff like “I will see you as soon as I’m done with him,” and stashing her phone every time her boyfriend/husband looked her way.
He explained on Facebook that he couldn't sit back and watch the deception without saying anything, so he decided to step in and pass a note to the man and alert him to the situation.
The do-gooder said: "Being the man that I am I couldn't help it… I had to write him a little note.
"On his way way out after the game was done, I simply waited for him to exit and stopped him in front of the woman and handed him a piece of paper.
"I said 'I don't know you and you don't know me, read this when you get home, TRUST ME.'
"He looked at me confused and said 'okay'…. I was a little tipsy but I don't care, it felt good.
"I hate fake people."
The piece of paper contained a simple message.
"I don't know you and you don't know me. When you get home check your girls phone. She's been texting "Jason" saying she wishes she was with him all day!
Take care. Wish you the best."
Nobody knows what happened when the couple got home from the football game, but many Reddit users have been busy imagining the worst.
And many have slammed the man for intervening without further details, insisting he could have put the woman in danger.
"He doesn't know what the boyfriend is like. He could be dangerous and abusive, and she could be too scared to leave him so she has a secret affair," wrote Reddit user Allan_Frankland.
There is also a question about the authenticity of the story, as the hashtag #ChiveOn was included at the bottom of the handwritten note.
"I think that whatever PR firm TheChive.com hired to stage and create this viral marketing campaign did an amazing job," wrote Reddit user mountainjourno.
What do you think - was this man acting in the right or wrong? Or do you think it's all a big fake? Let us know in the comments below.