I would not walk into this firestorm by telling anyone anything. I would advise your co worker not to do this and tell him that you will not be supporting this or getting involved. I would distance myself from him and your friend for a while. If you get involved, and he doesn't decide to leave her, then you will look like a complete jackass. Let them be adults. I would only tell his wife if she is truly in danger.
non of your concern, never intervent in anybody's personal life cause you might tell her and she might fight with him and then they make up and continue which will put you in embarrassing situation especially if the man convinced her that you are lying and doing that because you have feelings for him or any other crazy scenario !! your job as friend is comfort her later when she finds out don't get involved in anything else, use your time for your self don't waste it !
NO! You shouldn't tell her. Maybe it's just talk. Maybe she already knows but is pretending not to know. If you tell her she'll have to stop pretending. Telling her may well make things worse for her. There's nothing for you to gain by telling.