gavriel – I suspect you’re right. In any case, I think that this conversation has to come after other conversations, or it will sound condescending. (And it could be that was the Rav’s intention).
First he has to approach her and let her know that her dress bothers him (“I message” not “You message”). And he should make sure to point out why he thinks that it may not be appropriate, citing halachic sources. Then, if she is still having a hard time with it, maybe he can approach her again, and say, “I know this is hard for you, but I am very bothered by your dress. Would it make it easier for you if I bought you a diamond ring in exchange?”