My BGF has a crummy GF
Hey girl! I am really glad to hear that you are trying to be a good friend to your BGF. This is probably not the answer you want to hear, but there might not be much you can do to protect him from getting hurt. If he has already shot down your first attempt to help him out, he will probably only get more and more annoyed the more you try. I don’t think there is much that you can say to change his mind since he likes his girlfriend so much. Continuing to warn him is just going to make him mad at you.
I would say the best thing you can do in this situation is to let him do what he wants with his relationship and just be there for him if they break up, which it sounds like a possibility. You have already tried to be a good friend warning him. The next best thing you can do is to be there to help him pick of the pieces if he gets his heart broken. Whatever you do, don’t give him the ol’ I-told-you-so line. No friend wants to hear that when they’re upset.