kor ah.... my advice is, dun say anything yet. think of what she
will feel if you said anything. if she also has "feelings' for you,
she will feel torn between you and her bf. she might even sink into
guilt for her "emotional betrayal". if she didnt have feeligs for
you, it just makes things awkward, and what will she think of you,
telling a girl you like her when you knew perfectly she is
attached.
it's a lose-lose situation in my opinion.
just continue to be there for her. you may choose to still like her
despite the odds, or look around for other fishes. but whatever it
is, i will always advice against disturbing the equilibrium of
someone else's r/s..
good luck kor. 
