In the novella The Sworn Sword (not the graphic novel) ...
... the Red Widow is accused of burning down Wat's Wood (or sending someone to do it). She wants to prove her innocence with a trial by battle. The Red Widow's champion loses the battle, which means she's found guilty of the crime, but still there are no consequences.
Why wasn't she punished?
Actually KutuluMike is mistaken when he says that Lady Webber was the accuser and Ser Eustace was the defendant.
There were two main conflicts there:
There was also another conflict, which basically started the aforementioned two conflicts but it was in the sidelines as a result of more immediate events:
Ser Duncan resolved the first issue by taking responsibility for Ser Bennis' offense and inflicting the same injury on himself as Ser Bennis inflicted on Lady Webber's peasant, thereby settling that score.
The widow’s eyes went wide at the sight of naked steel. “What are you doing?” she said. “Have you lost yourwits ? There are a dozen crossbows trained on you.”
“You wanted blood for blood.” He laid the dagger against his cheek. “They told you wrong. It wasn’t Bennis cut that digger, it was me.” He pressed the edge of the steel into his face, slashed downward.
When he shook the blood off the blade, some spattered on her face.More freckles, he thought. “There, the Red Widow has her due. A cheek for a cheek.”
“You are quite mad.” The smoke had filled her eyes with tears. “If you were better born, I’d marry you.”
The Sworn Sword
After that, Duncan demanded that Lady Rohanne hand over the men who burned Ser Eustace's forest. Lady Webber had no intention of admitting that she was behind burning of the woods and even if someone among her men did it, she would not hand him over. She instead demanded that Ser Eustace withdraw his accusation that she burned his woods:
“Give me the men who burned the wood.”
“No one burned the wood,” she said, “but if some man of mine had done so, it must have been to please me. How could I give such a man to you?” She glanced back at her escort. “It would be best if Ser Eustace were just to withdraw his accusation.”
The Sworn Sword
At that point, Duncan made it clear to her that no such thing would happen. Lady Webber then chose to defend herself in a trial by combat in case Ser Eustace refused to issue an apology to her for his vile accusation, thereby making her the defendant and Ser Eustace the accuser.
“In that case, I must assert my innocence before the eyes of gods and men. Tell Ser Eustace that I demand an apology . . . or a trial. The choice is his.” She wheeled her horse about to ride back to her men.
The Sworn Sword
Later of course we know Duncan wins and Ser Lucas lost. The aftermath of the event is quoted below:
“Ser Eustace asked the boy to attend him at the wedding feast. There was no one else on his side. It would have been discourteous for him to refuse.”
“Wedding feast?” Dunk did not understand.
“You would not know, of course. Coldmoat and Standfast were reconciled after your battle. Lady Rohanne begged leave of old Ser Eustace to cross his land and visit Addam’s grave, and he granted her that right. She knelt before the blackberries and began to weep, and he was so moved that he went to comfort her. They spent the whole night talking of young Addam and my lady’s noble father.
Lord Wyman and Ser Eustace were fast friends, until the Blackfyre Rebellion. His lordship and my lady were wed this morning, by our good Septon Sefton. Eustace Osgrey is the lord of Coldmoat, and his chequy lion flies beside the Webber spider on every tower and wall.”
The Sworn Sword
So as we can see, Ser Eustace and Lady Rohanne shared an intimate moment together, reminiscing about the loved ones they shared and had lost which were Eustace's son Addam and Lady Rohanne's father Lord Wyman.
It is implied by the fact that they chose to reconcile, that Ser Eustace forgave Lady Webber for burning his woods, therefore giving up the right to exact punishment on her. They went even beyond just reconciliation, they tied their houses together. By doing that, they resolved their original dispute too, the one about the dam.
So, it wasn't a trial by battle which had no consequences. It was a trial by battle which ended in reconciliation, which is a huge consequence.
It's been a long time since I read that novella, but I think your plot summary in your spoiler is wrong.
As I recall, Lady Webber was the accuser in the confrontation with Dunk and Egg. One of her men had been wounded in a skirmish and she was bringing her army to get revenge. Duncan managed to talk her down into trial by combat between him (acting for Lord Osgrey, the defendant) and Lucas (acting for her, the accuser).
When Duncan won, he effectively ended the feud between them and made peace. She wasn't found guilty of anything, rather, her accusation that Osgrey had dishonored her was settled in Osgrey's favor.
