being afraid and remaining silent wont do any good. just bring on your guts, face her and tell her. ACCEPT OR REJECT, HAVE NO REGRETS. (don't give up even if she rejects)
i think the best thing you have to do is to think about it first, about what's more important to you.. is it the friendship that you and your friend had? or the girl who's already committed to your friend? and then you decide...if i were you i won't speak up anymore so no trouble will be created..but if you really love that girl so much and you are willing to take the risk then be it.. but just make sure that when you started it you will stand on your decision and face all the consequences that will come through you. but always remember that all of the advices that you will here from the people around you are just am opinion, at the end of the day you are the one to decide on what to do..being silent is not being coward...goodluck..