I have to agree with you, Laura, about the "once burned": it comes off as a real insult, and her barely contained anger lets loose. I'd probably let him have it, too.
Here's the line from the story:
"Once burned," he said, leaning against the counter, feeling too big for the room.
Ennis could have said something a lot more civil.
I think Alma was genuine in her caring for Ennis, and her comment about "me and the girls worry about you", was a gesture of that.
Then he let go with the "once burned" comment, which was like putting condemnation and blame on her.....and that was, I think, the straw that broke the camels back with Alma, as far as taking any of the blame for the marriage not working.She probably thought....."ok, enoughs enough, Im gonna let him know who and what did cause our marriage to disintegrate"......
Up until the time she spat out the "Jack Nasty" words, Ennis did not have any idea that Alma had had any suspicions about Jack, he thought he had got away with all of it, without her knowing anything, and he knew that everything she was saying to him was true. She was not only letting him know that she was aware that his "fishing trips" were a lie, she was telling him that she knew that Jack was more than just a fishing buddy to him.
The fact that she had held all that in for so long, is testement to her good nature....and also, I think she would have been afraid to confront him with it, while they were still married, because of his temper....the time had come, he lit the fuse with the "once burned" comment, and she exploded with the "Jack Nasty" one.........