It's irrelevant that Fred knew her parents were dead, it has nothing to do with the comment he made. It's a figure of speech and anyone with half a brain would be able to see that. She's made a mountain out of a molehill, comp
The realistic thing is, we have all lost someone be it friends, parents, Grandparents or even our own children. No one reacts like that. I thought Jamie had childlike attributes but, Jesus, Nella has petulant brat attributes.
She's shot herself in the food*. Fred is a bit of a national treasure and her outburst has had a domino effect (can't think of correct term I mean) and left a bad metallic taste, like a fuzzy tongue after drinking whisky all night.
Edit: *FOOT. Shot herself in the FOOT. Although food genuinely works too!!! Haha!