That's a tough question. I've exposed unfaithfulness a few times in the past, and I always struggled over the decision.
In all but one case, the person being cheated on sided with their partner. The cheater would claim that I was jealous of their love, or that I was trying to "steal" them, or that I had come on to them and was trying to get back at them for rejecting me, or some other ridiculous nonsense. Even when I had clear evidence, they claimed I faked it. Hence I became the enemy. No one was helped, and the only one who ended up hurt was me.
Although I would want someone to alert me to an affair, after my experiences, I usually choose not to get involved.
When I did get involved, it was because I cared about the person and didn't want to see them treated that way. I agree with everyone who says you'd be doing it for the wrong reasons.
Edit: I should add that I've also been in the situation where I knew about the cheating and didn't say anything, but ended up blamed anyway when the person being cheated on found out, and found out that I knew.
reynard_muldrake said:
Three questions:
1.) Do you think, years down the line, you'll be okay with this?
2.) Are you certain your claims will be taken seriously?
3.) What if no one believes you after the exposure of these facts?
I have no intention of telling you what to do/not to do. Just think things over before you act.
^ Agreed.