i will give u good advice, i hopee =]

well, first of all, she must consider u a good friend to admit that things r rough between her bf n herself... rite?
have u even told her your true feelings for her? does she know u like her to b ur gf?
if she doesn't know, i suggest u pour out your heart. but don't sound too desperate because it's not the right moment. i'll bet u anything that she's in a state of confusion about her bf.
admit u've fallen for her. it's best to do it face-to-face so she'll know that you're sincere n genuine about it. tell her that, if the relationship she's in now doesn't work out, there's always another guy who is willing to accept her n love her for who she is (n that's u!). dun expect her to make a decision fast. dun hurry her. give her encouragement n support. gurls usually fall for the guy who is there to comfort them n support them when they really need it.
if u don't keep badgering her to cut off her relationship with her bf n start one with u, she'll come to u eventually. ur job is to give her the support she needs. b her knight in shining armour from a distance n she'll come to appreciate u.