At the end, Heronia decides to confront Bertia to reveal all of her evil deeds (which of course don't exist) and condemn her.
But thanks to Cecil's plot, Heronia is the one who ends up being condemned, abandonned by everyone and about to be punished.
Her Light Spirit (the bird) then sacrifies itself to try and brainwash Cecil, which doesn't work.
Since her bird died, and since it was her only realy friend, Heronia gets depressed.
Cecil then gave her the remains of the Spirit and told her that, as a punishment, she was going to have to become a nun.
He then gave her two possible convents and told her to choose.
The first was one that was near the capital.
If she became a nun there, she would still be able to have a comfortable life.
The other is one that is located in an incredibly harsh and cold region.
If she goes there, she will have an incredibly hard and austere life.
However, that convent is also located somewhere that is full of Light Energy.
Which means that the Light Spirit might be able to heal and to be reborn (since she has its remains).
And so, since she wanted to be able to see her friend again, she decided to go for the second choice.
And that's all we know.
In the WN, I don't remember that we were ever told if she was able to see her friend again or not.
And I really want to know, because her end was incredibly depressing...