The entry in Merriam-Webster:
Nonplus (also nonplused; nonplussed also nonplused; nonplussing also nonplusing; nonplusses also nonpluses)
: to cause to be at a loss as to what to say, think, or do
: reduce to a state of total incapacity to act or decide : perplex, baffle, stump
<this turn of events nonplusses me — J. R. Perkins>
<nonplussed by the disclosure — Newsweek>
<for a moment the girl was nonplussed — A. R. Williams>
What really gets me is how the examples give absolutely NO help in reinforcing the meaning of the word. It's like I tell you I'm so green without you having any idea what green is.
Unless, of course, you are this guy. Then I totally know what you mean by green:
So, anyway, back to my point. My point is nonplussed is one of those words that might be misused because there might not be good context for the reader (or the writer!) to know what is really meant. I recently came across this exact scenario in a manuscript and I asked the author if nonplussed (at a loss as to what to say, think, or do) was really the word they meant to use because I could see the character reacting a different way.
I've had instances where an author used a word and I was sure the word shouldn't be used that way. But when I looked it up, there it was: some fourth or fifth (or sixth or seventh) definition that allowed for that particular word to be used the way the author intended. And I learned a new way to use an old word.
Guess those dictionaries (and those editors!) are handy, huh?