Reflecting on one's character's past actions isn't "being upset." In fact she used it as a dramatic tool, deciding her character would in fact feel quite bad over stealing a powerful magic item from an ally. Had Matt said nothing, she might not have taken the opportunity to consider that. She could have also decided her character was fine with it.Then she would be totally unreasonable for being upset by the change, right?
One thing's for sure. Matt wasn't telling her which choice was the only one allowed. I think Matt as an experienced DM knows how insidiously easy it can sometimes be for characters start doing suspect things here and there given their player knows all the characters in the fiction aren't real people with real feelings and real lives.