1) If they get back together, what are the odds that you won't be dissed and bitched about by them (for this)?
2) If they break up, on the surface either one (or both) may thank you for helping them "figure out" the truth, but inside, are they really grateful for what you did? The misery post-breaking up may exceed their capacity to notice how great of a friend you are and even if you did it out of kindness it might not matter to them, as it is trivial compared to the sadness and awkwardness they'd have to face.
3) The "Don't tell him/her I told you this" thing doesn't really work unless ur really lucky. So if ur really kind, u'll tell the one being cheated on WHAT YOU SAW (but don't add other opinions like 'i think he/she's cheating on you.' it's better to say what you saw/heard but not what you personally think). and when the cheating one confronts you, you can say
"you didn't tell me i can't say it and I did not lie."
he/she might answer: "oh but i wasn't cheating! it was a one-time thing, i was drunk! blablabla"
"ok but I didn't tell him/her you were cheating, i just told him/her what i saw. and he/she probably thinks u were."