Since I would want somebody to tell ME if they saw it, I would tell them, but only after I was certain that is truly what it was. Just seeing two people together does NOT mean they are "cheating" If you see the two of the in a lip-lock, or doing other intimate things, then I would go right up to the sleeze bag, and tell him what a real "cowardly man" he was, and NOT the stud he THOUGHT he was, and then I would tell up front, right then,, the next stop I was making was to his gf/bf. NOBODY deserves that. If the person isn't satisfied with their current bf/gf, they should have enough Balls to stop being a slimy good for nothing cheater long enough to be honest. Be free before you pursue another. A fact so many seem to forget now-a-days. I hope these dirt bags remember Karma never forgets.
Sometimes it's best to avoid these things. Unless you care for your friend a lot, I'd stay out of it. And if you really want to do something about it, I would get to your friends bf or gf right away and tell him or her what you saw.
I was once in this situation, except the individual involved was my friend's husband. There was no doubt in my mind that the jerk was cheating and I agonized long and hard over what I should do, I even consulted my wise mother. We finally agreed that I had to tell her. I did tell her, which was very hard for me, she cried, I apologized and said maybe I shouldn't have said anything, but she told me, "She needed to know these things." I think today with all the STDs around, we have to be willing to protect our friends from any risk they would experience from cheating husbands, wives, bf, or gf.