One Advice i've gotten a lot is do not hesitate.
Do you care about this girl much? Is she really a good friend of yours?
Are you afraid of loosing her?
If the answer to this is NO, then go man...
If you're straight shes gonna feel guilty (and we do not want that) and shes gonna put up her ASD.
Unless she acts slutty or really open mind about relationships, or unless she really really shows strong attraction for you, then yes, it might be a good idea to be direct.
Otherwise I'd recomend you to go some other way about it.
Try changing the topic and inviting her somewhere you two might enjoy, like you were friends, when you get the date, do as you would with a girl that's single....(Build attraction, kino, get her to qualify herself to you, kino, etc..)
If she does accept your invitation that it's definetely an IOI meaning that she might be willing to cheat on her BF. Thats a go for you my friend, and if you do get the date, please, Go for the Kiss close (or more if possible).
By the way. It's good that you know that even if you dont get the girl its still a good idea to make the experiment. There's no such thing as failiure when all that can happen to you is becoming wiser each time
