How do you explain nightmares to a lifelong lucid dreamer?

My wife is a lucid dreamer. She does not recall ever not being in control of her dreams, and has never had a nightmare. The entire concept is so foreign to her I’m having trouble portraying what it’s like.
I’d compare it to a horror movie, but she watches those all the time and is never scared. She learned from her father at a very early age that it’s all fake, so she’s never really phased by them. Why she watches them so much I don’t know.
So, so you have a description I can use? Any tips on helping her understand?

Imagine being IN the horror movie, and not knowing it’s a movie - it [seems to be] actually happening to you - you completely believe you are in genuine danger.

Have you ever been truly scared/frightened in your waking life? Imagine that feeling tenfold.
Edit: As a way of explaining it to an LD - A person who doesn’t LD perceives their dream EXACTLY like they perceive reality - Until they wake up.

So Imagine if you are awake, and (for example) you see a nuclear mushroom cloud a few miles away, and you know you only have seconds before you are incinerated by a 500mph wind of fire.

That happened to me just last week.

Can’t you compare it to any horrible situation where you’re not in control? A burglar in your house, being alone in a bad part of the city, etc? From there you can tell her that sometimes it’s like that, but with monsters added.

She did suffer abuse as a child. She often says she had no use for bad dreams because she was living in a nightmare.

For myself, the feelings of helplessness usually come only after I wake up. During them I’m mostly trying to make sense of the bizarre nature of what I’m witnessing or just plain scared.