Help, please, friend crisis!!

Err, who do you guys think you are? Her mother? It's her relationship, and her choice who she tells!

ok, so here is my problem:
one of my best friends hasn't told any of her close friends that she has been going out with one guy for about four months. Today, me and her close friends have found out that she had been hiding this from us. however, we found out by another girl, who isnt even a close friend. we are all very angry with her, because we always tell each other who we are going out with, and this makes us think she doesnt trust us.
i know that she doesnt have to tell us anything if she doesnt want to, but it has created a group crisis and i don't know what to do. some of my friends aren't talking to her (i know its childish) because if she doesn't trust us, they don't want to trust her either. on the other hand, other friends are going on as if this hadn't happened.
i don't think any of these positions is correct, so i explained to her that i was annoyed to have found this out not by her, but by another person. however, she has called me a liar, just because i told her my side of the story.

please, tell me what to do??? i just dont know, there are so many tensions between us, that noone is trusting each other. how can i save my friends from growing apart?

yes, i kindo know that, and i don't really care who she has been going out with. the main thing, is that it is because of this that we have stopped being friends.