I've known people in this kind of a re-lationship. And in my experience, there's almost nothing you can say or do to change their behavior.
The BF -- he thinks he is doing your friend The Greatest Favor In The World by gracing her with his company when he feels like it.
Your friend -- wants his "love" so desparately that she puts up with this, not realizing that what he is giving her isn't love at all.
When I've been the friend in this kind of situation, I've found that just completely backing off is the best thing to do for me. I end up just telling the friend, "Looks like your really busy with your BF and that's fine, so I'll put the ball in your court. If you want to hang out, just let me know and we'll see about making plans then!"
Inevitably, I don't hear back unless and until they really do break up. Yes, I've lost friendships that way, but really, it's not much of a friendship if one person is treating the other one this way.
I think it's probably worth your talking to her about this as you've planned to do, for the sake of your own life experience. Sometimes we just have to go through an experience to really understand how it works. But my advice is to go into this with your eyes wide open. Expect the worst. Do not expect or even hope that your friend will suddenly realize what a jerk BF is and how she is in the wrong for not only allowing him to do this to her but for allowing him to make her do this to her friends. If I'm wrong, so much the better. You'll be pleasantly surprised! But if I'm right, preparing for that will at least allow you to avoid having your feelings hurt worse than they already have been. Believe me, I've been there. I've felt that pain of thinking that years of friendship would at least have some influence but when it comes down to it, that desparate desire for love and acceptance by the BF pretty much wins out every time.