A friend caught his wife naked with another guy recently

And he doesn't want to address it at all:

Friend lets New Dude move in with him and his wife. New Dude doesn't pay rent at the time because he's "trying to get on his feet." I think it's cool that they are trying to help New Dude out, especially since he doesn't speak English as a first language and doesn't have any marketable skills. Last week, friend told me he walked in on New Dude and his wife in the bedroom and they were both naked. He really doesn't seem to want to do anything about this. I brought up how f'ed up this is a couple times and he seemed uninterested in what I have to say, but I can tell he's been losing sleep lately and he's clearly losing a good bit of his paycheck to support New Dude. New Dude seems like a cool guy but I think it's messed up that my friend is going through this. It's only going to get worse. How should I approach my friend and get him to listen?

EDIT: I found a picture of New Dude to give you a better idea of the situation: http://imgur.com/NpCur.jpg

(reddit)