As far as the poll goes, I'm down the middle. Seems to me you are both are in violation of the one rule that governs these situations: Bros before hoes. Yes, he should have told you told you what was coming down the line. Especially given the time frame. Since it's not spelled out here, I understand this was kept secret for some 8 to 12 weeks. That's pretty fucked.
However, you must also concede that your woman put you both in this shitty situation and essentially jeopardized your friendship on top of the breakup by asking him to keep it quiet. I know it's tough to find fault in the woman you love, even if she breaks your heart, but she unnecessarily placed you both in a very shitty position bringing him into the middle of your own relationship. It wasn't really any of his business in the first place.
My guess is that, whether consciously or not, she wanted him to tell you so she wouldn't have to deal with the confrontation and end up being the bad guy. If he tells you, then it places the need to address the issue in your court and she doesn't even have to bring it up. Except that's not what happened because she made him "promise" not to tell you. And said friend, a man of his word, actually kept his promise.
So now here you are breaking the bro code by placing her above your friend due to the way she manipulated you both. Again, I think it was unintentional on her part, and she absolutely would not wish the current outcome. But she ultimately comes out of this unscathed in comparison. My advice is that you should both apologize to each other for being shitty, have a beer, and sock each other hard as fuck in the arm and get over it.