Should I tell a stranger that his girlfriend cheated on him with my ex?

Dear Starshine,

I began dating someone — we'll call him J — in May 2012. We were on and off all summer and fall trying to figure out what we wanted in a relationship, and finally ended up together. Then I saw a text from another girl (M) indicating he had slept with her. He didn't deny it, but because it happened during an "off" period, I let it go. A few months later, I discovered that he still had naked pictures of her on his phone — even after we had decided to be a couple. I felt stupid, disgusting, and used. We're no longer together for lots of reasons — but I found out that M has a long-term boyfriend (S) who doesn't know about any of this, which brings me to my question: Should I be a good person and tell S that his girlfriend cheated on him, or be a good person and keep my mouth shut — even though I'm innocent in this and was the one who ended up getting hurt? I don't personally know them, but have contact info for both. Do I tell him? Do I tell her that I know and that if she doesn't tell him, I will? Or do I stay out of it and let S continue wasting his time with a trashy skank?

Ooh, and you almost had me, too. So close! I thought you might be genuinely concerned about the emotional welfare of poor, cuckolded, blind-sided S. You were innocent. You were stupid, disgusting, and used. You were eager only to spare this guy the heartache that you suffered by arming him with the facts surrounding his faithless, er, photogenic sweetheart. The truth shall set him free, knowledge is power and all that.

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But then "trashy skank" tumbled out of your still-fuming face at the last possible minute — and the jig was up. Darling, this ain't humanitarianism. This isn't about protecting your fellow forlorn fool-for-love. This is revenge, pure and unattractive. You want to make sure nude-photo lady can't have two guys when you can't have even one.

So, yeah. Go for it. Call up a stranger and try to sound like a reasonable, even charitable person while telling him that his girlfriend is a slut and that she slept with your man, whom you are no longer dating in part because he likes the way she looks naked. (Is anyone else hearing Ross on Friends tell Rachel, "WE WERE ON A BREAK!"?)

Just promise me one thing: Once you make this gotcha call, you'll stop referring to yourself as "innocent." You lie down with dogs, you get up with trashy skanks.

Send me your dilemmas via email: ToughLove@TheWeek.com. And follow me on Twitter: @ToughLoveAdvice.