Is this some manner of idiom? I certainly haven´t heard this phrase in English.
Maybe "and her threatening look" would be a better translation
"amenaza" comes from the verb "amenazar". The adjective "threatening" would be "amenazante".
why is it "her" and not "his"?
You cannot know that from the form. You can only know that from the context (ie: whether you were talking about a boy or a girl). So both "his" and "her" are correct.
Thank you - Duolingo did not accept "his"...
It did for me this morning, so they have already changed it :)