The title is a bit more straight forward than it really is.

I have known my buddy since middle school (we are 35 now) and he has always been a serial cheater. Over the past about year or so he has been hooking up with his secretary. I have known about it the whole time (he likes to brag). Up to this point I have never told his wife (or any of his previous GFs when he was cheating on them), it was just one of those things where I said, "not my life, not my problem"

Well last night we got into an argument while we were at a local brewery. It started to escalate and he started throwing in some low blows that really pissed me off. So purely out of anger I say, "At least I am not banging the secretary behind my wife's back!" while his wife was sitting right next to him.

Telling someone they are being cheated on=not asshole move. but does the context change that?