Should You Tell A Married Man's Wife That His Husband Has Been Cheating On Her For 2 Years?

It is hard to comment to this. To be having an affair, who is in the room that knows this is happening for sure. If you have know, or thought you knew for two years, what has happened that makes you now wonder if she should know.
e world she chose to stay in.
If she is a great friend a
Chances are she already knows, and just want to stay in the marriage with hope. Sometimes people will get angry with you for telling, even if it is true. If she knows and you tell her, you will have rocked thnd she felt something was not right, and she asked you if you know anything, then I suppose I would tell her about gossip. Don't mention how long you have known.

I think that you should mind your own business and stay out of it.

I think the same with Eleanor Jones

Oh my gosh if you knew for so long you should have told her the moment you found out   even if it breaks her heart it would kill her that you knew and never told her she would probably not speak to you for along time and would you rather her find out herself

If the person that he is in the affair with is you don't do it personally at the same time if it is not you be very careful about it because if this is a close friend and you care about them when it comes to their husband it can be very tricky she might not believe you and it could end very badly for your friendship good luck

No, never interfere in another persons marriage.

Yes its not fair to any one that is being cheated on 

Tell her. Let her make her own choice

You know, I am not so sure you should say anything to HER.  But maybe if you confront HIM, telling you know about the affair, and he'd better break it off, or tell his wife, or you WILL tell her.