I agree with Right On Time. That's akin to what Cheater, the TV program does, though they build up their case first to have enough proof of infidelity. In situations like this, one incident isn't enough.
Also you mention "know"...how did you find out? If you've seen his wife with another man (and she doesn't know you or remember who you are), perhaps you could take some photos of them being affectionate with each other using your cellphone or ask a companion to do it for you (in case the wife knows you). Be careful though so as not to make them suspicious of you.
Another way is to ask your friend if he's noticed some changes in his wife's behavior - gets irritated when he asks her questions, spends longer hours preparing herself before she goes out of the house, less physical intimacy, doesn't appear concerned if he spends days away from home working on some project. These clues also go with philandering husbands.
Drop some hints if you can - hopefully, your friend's sensitive enough to know something's amiss. You could suggest that he might want to try telling his wife he's going away some time and then return unexpectedly. He might see something he likewise never expected.
I just hope he won't take it against you that you're helping him find out the truth. It's a different story though if he's aware she's having an affair with someone. There are people who are okay with that - as long as they know about the relationship beforehand.