Several months ago, in a private message to someone, I quoted a specific rav to someone, and she wrote to me “He Isn’t a real rabbi” even though he has man haskamas from talmedi chachamim. This person has a rebelliousness nature in general and I’m pretty sure her own rav doesn’t typically rebuke her very much probably because of Mutav sheyiheyu shogegin, although I’m not 100% sure. In any event, since she disrespected the kavod of this rav, was I obligated to protest, even though she doesn’t respect him (or many rabbeim she doesn’t agree with). Do I need to do it now, after the fact, if it has been some time afterwards?
From what you are describing, it sounds like telling this person will not really accomplish anything, and it even have a decent chance that telling the person something will cause the person to be even more mezalzel in the Rov. The best you can do is to protest what she did in front of others, as a way of partially saving to honor of the disrespected Rov. Therefore, you can tell people when the opportunity arises that you once heard someone say disrespectful things about Rav…, but she unfortunately doesn’t have proper kavod chachamim.
This seems to be the best that you can do in this situation.