What you have to do is make sure she knows that you think she's "pretty" or "cute" and that you're "interested". Do this by telling her friends, guaranteed it will get back to her! Once the girl decidees to hang out with you and already knows this, it lessens her odds of rejection to close to nil. This will make her feel more "secure" and more "comfortable" b/c she can act more herself and doesn't have to keep her "defensive shield" up b/c she knows that you probably won't reject her.
Other tips are: When asking her out, don't classify it as a "date". This way, in her mind, she's not cheating on her man. Just say, let's go out for fun or just to chill. Technically, if she says yes, both of you will know it's an informal "date", but women like to leave things undefined in case things DO work out with their man (erases their guilty conscience knowing it wasn't a date). Make sure you act casual with her, and smile alot/make her laugh (obvious things). Don't bring the b/f thing, let her bring it up. Most importantly, if you're feelin' the vibe, kiss her at the end of the first date. Even if she pulls away, she knows that you want more than "friends" (trust me, you don't want to be stuck in the friend zone!!!). Of course, this is subjective, you have to go with your instincts here. Either way, go for it. You only live life once!