Hi Laure,
I would confront the guy.
Say, “Listen, it’s technically none of my business, but a dozen people obviously know what you’ve done. It’s a small world and life is long. Your girlfriend is bound to find out you’re a cheat one day if you keep this up. What’s the deal, buddy?”
If he replies that you’re right, it IS none of your business, say, “Well that’s the thing. It’s so blatant that it’s affected everyone’s reality. Like pollution. Buddy, it’s unlikely we will run into each other again, buy if I catch wind that you pulled a stunt like this again, I WILL tell your girlfriend and get this over with. You’ve been warned.”
Maybe that’s the kick in the pants he needs to behave or be discreet.
Best,
Inky