i don't see a critical junction coming anytime soon. you barely know the girl. just have fun with her, get to know her.
generally when you're pursuing a girl for romantic reasons, you want to make it obvious from the start, otherwise you end up in the friend zone. but in this case, she's already got a bf. hitting on her would just be plain tacky.
as far as the terms of their relationship, you don't know what it's like. maybe it's a loose bf she never really went out with long term, maybe they've been sweethearts since birth. regardless, you'll learn in time. so stop worrying about it and just have some good times until then.
who knows, maybe she knows some hotter friends
