As long as when your friend asks what he said about considering a relationship with her, you tell her the truth. BUT, be sure to include the fact that the two of you (you and your neighbor) have been exchanging messages for about a month now, and that the two of you might get together. That way, that leaves room for you to tell him how you've been feeling, and the MIGHT part also leaves room for nothing to happen between you and the guy, if nothing happens to come out of it.
So, don't worry too much about betraying her...IF you're up-front and honest with her from the start, there should be no hard feelings from her at all.
