2

Nighteye was about to die and they had a girl who can rewind someone to their former state, make them younger or repair their injuries.

Why didn’t the heroes try to use Eri to rewind Nighteye to prevent him from dying?

3

There are multiple reasons behind this:

  1. Quirk. As @Kerkhof & @ConMan said, she can't control her powers and could do more harm then good. We still don't know the extent of her quirk so she could turn in into a baby in 3 seconds if she loses control maybe.

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  1. Eri's state. After the fight with Chisaki, she is seen in a deep sleep/"coma" like state and they could do nothing to wake her up so she could'nt help even if she wanted to.

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  1. Time. We see in chapter 161 that Night-Eye dies moments after they are all rushed to the nearest hospital so even if Eri was conscious and healthy and even if they were able to find a way to use Eri's powers to help Night-Eye, it would've been a race against time and there were no assurances that they could do it.

  2. Trauma: Everyone in knew how much Eri suffered. And how she blamed herself for everything. Seeing this people decided not to tell Eri about Night-Eye's Death. Because she is too small to understand/process all this. She might have even started hating herself or go berserk.(context: Anime)

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2

As Eraserhead says when he's escorting Deku to Nighteye's room, Eri's Quirk is not under control and is likely to destroy someone rather than heal them, which is why he says "We cannot depend on her Quirk" - specifically, they can't make use of it to save Nighteye.

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0

I believe this is because they put her in quarantine. With the reason being that she can’t control her power, which could result in restoring him to nothing.

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