“Otherwise she may be teaching her children that her subjectivity overrides. Chas v’shalom she may be adding to the Torah. That’s also a transgression.”
That would only be true if the Rav told her that she HAS to stop covering her hair, or if he told her that according to halacha, there is no reason to cover her hair.
My impression is that if and when divorced or widowed women are told they don’t have to cover their hair, it is based on a leniency, and they are only being given that answer because they ASKED the question and they were asking for a leniency. If they had phrased the question differently, “Is there any reason for a married woman to cover her hair?” or “SHOULD I cover my hair?”, they may have received a different answer.
In point of fact, I’m not 100% sure, but I believe that all or most poskim say that you should cover your hair in such a case. Although it’s possible that in specific cases, when it’s really hard for the lady,etc. they may find leniencies.