I had to go into my Nvidia settings on the desktop I built for my girlfriend and turn up the exposure, It was difficult to pin down because it wasnt the brightness, if we turned up the brightness everything just started to get washed out with, not white, but brightness? I didn't make it easier to see. It was when I went into her NVIDIA settings and saw exposure turned all the way down by default I checked my PC (built this one exactly the same) and noticed my setting was like 35% or something so I changed hers to match mine and thats how we fixed it. Hope it helps.

It's been mentioned before; the game needs a gamma setting, and a calibration step during first launch.
It's standard practice for games that make significant use of dark scenes.
Simple enough to implement, but understandable to be missing in EA.

try Reshade to make valheim on your girlfriends computer brighter, but that is not a fix. To fix it you can try reinstalling vram drivers and the game too

First of all, what's her laptop's screen? Cuz it makes a difference on what tech it's made.
Cheaper screens usually don't have such a deep colour depth that the game can display it properly.

Second of all, what are the gpu's software settings? Depending what preset she has set, be it TV, Gaming, Dayligh, etc. the colours, contrast or gamma can be set differently.
And yes, presets can be either set as global or per application. Not to mention, some settings can be set to override the application settings.

There can be multitude of causes.