i'm exploding and want to smash her face,, HELP?

[Bonbon Bonbona]

soo! my best friend has told the guy i like every secret i shared with her about him, told him i loved him and how jealous i get sometime,, every single thing.
she asked him not to tell me, because she did this one time before and i got mad but we passed it.
and i told her last time that i hated that and asked her never to do it again, but this time she told him every sing detail xS
and now i'm furious and would very much like to smash her face, but problem is i shouldn't have know
i knew it by chance i logged in to his facebook account and read the conversation.
so i can't smash her face without telling that i went through his account
but i'm really really angry to act like nothing happened
i need advise people!!
can i just tell her that someone hacked into his account and sent me the conversation, or what the hell should i do!! ??
HELP!!