Someone mentioned that uncovered hair may help with shidduchim.
Maybe the rav told the woman that she wasn’t required because in her case she wasn’the required. Plus enough time passed where she felt finally ready to shed herself from mourning her late husband of beloved memory and find a new husband.
In that case the chinuch could still be consistent. She wanted to honor her late husband, after his passing, by covering her hair for him, which she had not done when they were together.
Then as the mourning lifted and she learned that she did not have to cover her hair according to the rav, she decided to leave it uncovered to be open to remarriage and maybe finally covering it again with her next husband.
If she listened to a rav and if needed got a second rabbinic opinion to be sure, and they said that she didn’t need to cover it, then she would be modeling the importance of listening to one’s rav.
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.