Where is the border?

My friend did something yesterday that I'm not sure how to react to. She told somebody something that she knew I wanted to keep quiet, and wasn't anything to do with her anyway. But should I be angry? I mean, it meant that I plucked up the courage to tell my other friend, or more talk about it with him. But I don't know how I should react. What she did was go against my trust as I confided in her. But, she only did it because she was worried about me. So what do I do?