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.

I asked her to check whether she had the TS4_x64 file/application, but she didn't have a game folder in the sims 4 folder. Does this mean her game isn't installed? I've attached a picture of what she does have in her TS4 folder.

@Lulutim16  That's the user folder, not the program files.  Your friend should be looking in a location something like this:

Program Files (x86)\Steam\steamapps\common\The Sims 4\Game\Bin

or

Program Files (x86)\Origin Games\The Sims 4\Game\Bin

She’s found the file, but the pop-up is still asking her to install it again. If she clicks install, will it verify the files and just proceed to open the game? 

@Lulutim16  It should, but there's only one way to find out.

I'm not trying to be flippant, just saying that testing will be more accurate than my guessing.  This isn't a common error, so I can't tell you how it generally works.

@c4thhhhhh  If you're being asked to reinstall Sims 4, you can simply choose the location where the game is already installed; the EA App should verify the files instead of redownloading them.  This might work for Steam as well, but I don't own the game in Steam and haven't heard from others either way.

If that's not the issue, or you can't reinstall, please create a new thread and describe what you're seeing in more detail.  A screenshot or two would be helpful as well.

Since this thread is somewhat old, I'll lock it here.