As much as it will be eating you up, that he doesn't know? It's not your place to tell him of this. As much as you may dislike the g/friend too, you need to encourage her to tell him. Keep on it until something is done, but not by you - make sense? Otherwise you will be seen as the bad person...when you're not.
I would tell him right away, or when you want to, because there is never a good time to tell your brother..or anyone that there gf is cheating on him. Plus, if you were being cheated on wouldnt you want to know right away?