She broke her promise, but she can't know I know.... HELP?

Okay.. My friend gave me her facebook pword to say weird things to her boyfriend so he'd break up with her or something, I don't even know, but she begged me to go on it, so I did.. she though I had forgot her pword, but I didn't.. so I just went on it, ( I know, call me nosy ) .. We promised eachother that we wouldn't drink or do drugs or anything like that, but i found out she drank with her other friends a little while ago, so she wrote down on a piece of paper " I ____ will never drink any type of alcohol " and this was HER idea to write it and she signed her signature...... She promised, I mean... She's my bestfriend, I don't want her drinking. And plus SHES the one who promised... anyways... I know, I know, it's wrong.. But I clicked on one of her emails on facebook, and well..... her friend " J " sent her an inbox that said " if i can get alcohol this weekend want to get drunk? :) " and I scrolled down and " C ( my friend " ) just put a bunch of ' smiley faces ' and " J " replied " I'll take that as a yes ! :) " and " C " put a nother smiley face, It's not even about the drinking, well it is a little bit, it's the fact that she PROMISED me something and then just went behind my back... I thought we were bestfriends? that didn't hide anything or TRY to lie, i'm very hurt that she didn't say " no, i'm not drinking anymore " but i CANT let her know I'm mad at her because then she'll know I read it.. I just, don't know what to do... Btw, she's only 14.