Personally being a bit obsessed in the area of personal integrity (maybe my INFJ trait ;) I think the most powerful thing you did was admit to yourself that you were out of your own integrity then took the next step to step away. This must have been very hard for you but even harder for someone who has integrity is living outside of it. It feels like a split from the most important human partner you can find - the self.

I wouldn't know what to do in your situation as I can't say I've had the same experience but when I do find myself living out of integrity I try to do what you've done, first take the hit and admit it, second step away from the situation and third re-balance myself before taking any action in the physical world. Most often I use meditation and imagining to see the situation working out (in its best way) and focus that all unfolds well for all parties involved then like you, get on with focusing on working on myself.

Taking this internal journey can be even more powerful then actually confronting someone (at least doing it first anyway). When the inner balance comes then a natural undramatic answer will most likely lay its self at your feet and/or the situation between them may simply take a new dramatic direction that has nothing to do with you. Sounds like the guy and his girlfriend are not new to their co-dramas so may simply chose to continue it no matter if you speak or not. Good on you though for stepping back into your own center.

It sounds like you admitted your error. What else COULD you do? I think that is enough. Bringing it up again only perpetuates the conflict. I would suggest letting past mistakes go and move forward.