That's a strong diagnosis. It does not seem fair to have heard that from her brother, believe it, and not have proof. Why didn't he mention something along those lines before now? I am a firm believer in 2 sides to every story. I have been the victim of the "other side," where my account did not get to be told. I'm not saying you are wrong. I know your intuition is sharp. She may have felt that vibe and acted strange, knowing you were seeing through a lot of her. It's always blissful at first. When a relationship starts taking work, the ball drops; either way, your friend will have a lot of healing to do. He is fortunate to have you.