Well, I have almost the same situation. A co-worker asked me today why her face is so greasy and what should she do to have a normal skin. What should I have told her? That she should try to watch her diet? Yes, of course. But I didn't. I think that she wanted to hear something else. I only tell my sincere opinion if the person asking me is a close friend and knows that I mean well.
I agree that Betty overreacted, but also, Julie could avoid to answer to such a provocative question, I suppose she knew how Betty could react. I would have answered something else, not stating my opinion, probably.