If you tell this person that you are angry, there is no way to explain why you are angry unless you break your promise. On the other hand, if you don't tell this person how you feel, you will be guilty of the same thing they did to you (minus the talking behind their back part). If you want people to be straightforward with you, you owe it to them to be straightforward yourself, and it sets a good example for them.
If I were you, I would break the promise (explain to the friend you promised first), tell this person who was doing the project with you how you feel, how you found out what happened, and explain how much it upsets you when they hide stuff from you, and that you don't want a relationship like that.