Re: Sims 4 asking to install again after already installing- Is this an update?
@Lulutim16 Please have your friend check that the game is still installed, as in, look for the program files and check the size of the Sims 4 folder. Inside ...\The Sims 4\Game\Bin, please also see whether there's a TS4_x64.exe present.
If the program files are present and Steam's default install location is still the same as where the game's program files are located, then clicking Install shouldn't be an issue. Steam should start to download, verify the presence of the files, and finish.
By the way, your friend will need to clear a bit more space on her hard drive sooner or later, for example when she wants Windows to do a feature update. That requires 20 GB free unless you use a USB stick as an install medium.