Your opinion with explanation pls

I don't know what to do. So my sister's boyfriend like has a bestie, a girl bestie. And she is like always around him, and its painfully obvious that she is into him or else why would she be third wheeling most of their dates, interrupting their nightime calls even if it's midnight. And the hilarious part is I don't know if my boy is encouraging her to continue or if he's just stupid. When my sister confronted him he said he had known her his whole life and she struggles a lot due to her family issues, her past break ups that emotionally scared her (and I be honest her past relationships was traumatic but it's been freaking two years) maybe she is the kind to heal very slow. Anyway he says that he should be there for her and that's his responsibility. So my sister stuck between should I break up or what else am I too do. And let me say, she truly loves him it's painful to watch 🥲🥲🥲🥲🥲🥲🥲