Having seen this happen in real life with a close friend, I say you get involved, assuming you are close with the victim.
One of my dear friends discovered her DH was cheating when a mutual friend spotted him on the news, in the audience at a local NHL game. He was holding hands and smooching with another woman, it was pretty obvious they were not "just friends." I truly feel for this friend and am so glad I was not put in this situation, because she struggled for days, questioning what she saw, etc. She finally went down to the station and asked for a copy of the newscast. When she got it she was able to study it and confirm it was indeed our friend's husband. She finally took the tape and told our friend, who was obviously devastated. And naturally her husband denied it until she walked over to the TV and popped the tape in...busted! Then he starts claiming he never knew love until he met this other woman, all this mean, hateful stuff, and up and left her. Leaving her alone with a 4yo, 20 mo old. and 12 month old.
Long story short it just about killed her. She spent almost an entire summer in bed from depression and if not for her friends and neighbors the kids would not have made it either. But she did come out of it and today things are a million times brighter in her world. She is still a single parent which is HARD, but I asked her if she ever regrets M. confronting her with the info, and she said NO!!! She is glad she found out, is glad the sham didn't go on any longer, and in the long run knows she is better off without him (although her heart still breaks for her kids, who he really could care less about).