Etiquette wise, I think asking your BF's friends about him/your relationship without his knowledge should be reserved for situations where you are generally worried about *him*. I'm thinking of something like a mental health issue that's cropped up, or you're worried that he's so stressed that something needs to give, and ask his best friend "I haven't seen BF this stressed - you've known him longer than I have, should I be worried?", that sort of thing.
I know that if a SO of a friend of mine came up to quiz me about my friend's relationship status with them, I'd feel very uncomfortable. I definitely wouldn't answer questions like that, and I'd regard reporting on conversations we've had about her as a betrayal of the friendship. Depending on what they were asking, and the closeness of my friendship, I might even feel obliged to tell my friend about it.
And thinking about it, if I found that my SO had been asking my friends about the status of our relationship, and had even asked them to tell him about conversations they had had about our relationship, instead of talking to me about it, I'd consider that a serious invasion of boundaries, and would probably break up with them immediately.