Would you tell? Watch
For those that can't be bothered..
TLDR: Had a 9+ casual relationship with someone who I've now found out was and still is in a relationship at the time. Would you tell their partner that they'd been cheated on?
- Study Helper
You should talk to him about it, and directly confront him on it. He has to be the one to admit it, coming from you will make everything a million times worse for everyone, especially the one in the relationship with him.
Although things could get hairy.
Do you know you will be safe if you do tell her? (ie, he isn't psycho and isn't going to track you down for it)
I don't think you have any obligation to do it, but at the same time, it isn't as if you shouldn't do it, so I would leave it down to personal choice.
Either way, you need to talk to him and it's better if it comes from him and not you.
No one can blame you for seeking retribution and comfort so you can move on.
Edit: Of course his partner could already know about you in which case your whole plan will fail so speak to him first.