Do I just come clean and tell her what's happening or keep it to myself ?
Well, you should come clean and then if she can forgive you, you've got a good friend there. if it was me, I'd cut you out and pretend like you never existed. i'm sure you feel bad about it, but you should feel worst, its one thing he cheated on her, we girls expect that from guys, they're jerks..but our bestfriends? no ..you should know better.