I don't want to bust your heart shaped bubble, but dude, you are asking for help to steal some other guys GIRLFRIEND.
You hang out with them, and the whole time she thinks of you as a friend, and he thinks of you as in the way. No wonder he doesn't like for you to hang around, he picks up on the vibe that you're giving off. The smitten vibe.
You say that she has to be there for him, guess what? That's what people do in relationships. Whatever problems they have, is their business. You are using them as a wedge to inject yourself into her life.
I really don't want to be rude or harsh with you, but this is crossing the line. I believe you when you say you are naïve, and I don't want to see you get hurt. And by hurt, I don't mean emotionally. This guy may come after you with a ball bat if you don't watch it.
You think that she feels the same way too, but has she ever tried to kiss you, or even imply that she would like to be with you?
Go places where you can meet SINGLE girls, not necessarily bars either.
If one day they do indeed break-up, then there's your chance. But for now, she is off limits. Would you do the same things if she was MARRIED?
I wish for you luck and CONTROL.
You asked for suggestions, and there's mine.