['I just discovered that my girlfriend had been sleeping with my best friend', "That's terrible!  How did you find out?", "My friend confessed which, I give kudos for, but he knows what he did. It doesn't make it ok"]
It's hard to find forgiveness for something like that.  Still it's better that he confessed than you having to find out otherwise.  If only a little better/
I just discovered that my girlfriend had been sleeping with my best friend. It's infuriating to know that she did that behind my back