I still don't think I would. I don't know the guy. I don't know if they have a Don't Ask Don't Tell policy. I don't know if the information might cause him to become violent (I tend to believe that even adulterers don't deserve that). I don't know if he already suspects but would really prefer to keep up the plausible deniability. I don't know if they are stupidly happy together and he would resent having that shattered. I don't know if she is a good liar who will blame the "obvious lie" on an angry ex. I don't know the guy.
My grandparents slept around like really dysfunctional bunnies, and every reveal just led to a huge fight that scared their kids, followed by a return to status quo until the next big fight. I find it hard to argue that either reveal did any good for anyone.
I mean, I get the argument. I'm reasonably sure I'd want to know (but then, I haven't to my knowledge been cheated on, so what do I know). I understand the impulse. And maybe the odds of a good outcome are high enough that it's worth the risk.
But without knowing the guy, I'm really not sure I'd do it.