I ended up telling him that I didn't want to break the news to her about the trip and he told her himself.
As for continuing to date her, we have already decided to get together on our own without even telling him. My concern is still that she is expecting more from him (she clings to this hope that he will someday want more from her than just sex). I feel bad knowing that this will NEVER be the case. In fact, he has told me that he is planning on cutting her out of his life altogether. What then?