For what its worth....
You didn't see what happened. You have a rumor of bad behavior, not a personal knowledge. As for kissing....there are a lot of scenarios that could be completely innocent. Maybe it was a kiss of greeting, or goodbye, or a kiss between friends. Lots of people have different boundaries for that sort of thing. Because you did not see what happened, if you talk to your friends, it will just be relaying possibly hurtful gossip. I'd keep my mouth shut, and my eyes open. If you do see something, then you can re-evaluate.
At this point, you know the first friends are not people you should trust in a delicate situation. They seem to be fairly free with spreading rumors and stirring trouble. The "cheater" should be given the benefit of the doubt until you have a real reason to believe otherwise.
I also disagree with most here. If you, personally, have witnessed something, I feel that the obligation of friendship would be to help a friend with the situation. That would mean your dh approaching his best friend with the evidence, and asking about it, perhaps in a "do you really think this is the best idea" sort of way. The only obligation of friendship you have in this situation might be to inform your friend that there are some nasty rumors floating around.....