Hmmm. A drink isn't an affair. "Colluding" is kind of strong language for keeping quiet about something that may innocent but might upset you. I subscribe to the Snake Discovery channel. Are we 'colluding' when I don't mention it to my partner because snakes frighten the bejesus out of him?
If your ex says they did have an affair, you'll be angry. If he says no, you may not believe him, and you'll be angry about that, anyway. Either way, since it's in the past, it's irrelevant.
Does your friend still have affairs? I'd give her wide berth, in that case. And it's not *just* that she has them. I'd be puzzled, (okay, put off) by the fact that she sees it as so innocuous that she TELLS people about them. It's one thing to do it once, feel guilty enough to keep it a secret and stop. It's another thing if it's some kind of lifestyle choice.