(1) I should have said "Installation media". By that I mean a USB or DVD media that has the Win 10 Installation app on it from...
https://www.microsoft.com/en-us/software-download/windows10
Download Windows 10 -- currently, this is version 2004
That can be used 3 ways: Clean install Win 10, upgrade/repair install it, or to boot to its own copy of the recovery environment (Advanced Startup).
(2) That URL I posted earlier...
https://support.microsoft.com/en-us/help/4000735/windows-10-reinstall
Reinstall Windows 10
...shows the various ways Windows can be reinstalled & what happens with each one. Click the various methods one at a time up top, & an explanation opens for it at bottom.
(3) "The options are only whether to keep all files and applications or not."
That definitely is not a clean install which wipes all files (except from OneDrive) & apps. It must have been an upgrade/repair-install. That is the only one that offers to keep apps. A Reset will offer to keep only files. Read carefully through that URL
to see what they do.
(4) "by repair/install, maybe you mean"
An upgrade/repair-install is what I said. Boot to windows, plug in the Installation media, open it, & double-click Setup.exe. It is a repair, if the media is the same Windows version as the current installation. If greater, it is an upgrade. (Lesser should
not be attempted.) This is the only reinstallation method that will offer to keep apps (as well as files). A Reset offers files only, if even that.
(5) "The installer doesn’t delete partitions"
If you do a clean install, you have the option to delete partitions at the "where do you want to install Windows" screen. Don't delete one she may want to keep, but only delete EFI, MSR, Windows, & Recovery. Click next, & they all regenerate clean, but OneDrive
should survive.
But you may choose to delete none, just select the Windows partition & click Next instead. In that case, you get a clean-ish install but a Windows.old folder will be created with her current installation inside. Here files will be inside & could be fished
out. I'm unsure, but probably EFI is repurposed & Recovery is regenerated, maybe you end up with two.
(6) "The issue I ran into is that the installer will not install to any partition other than the one it is running from."
I don't understand that. The installer is on separate media.
(7) "Something seems to be missing between #2 and #3."
"I see nothing to pick a device to run something from."
I edited stuff out realizing her computer couldn't boot to do that. But the URL has it all. But to see a device there, it has to be plugged in (or inserted) before getting there. I know for sure USB drives will show up. If you insist, I'll try it with a
DVD, but I think it should...

(8) "The real issue is that its stupid to make you boot to the drive you’re installing to"
That's only for an upgrade/repair-install. A clean install can't be done that way. A Reset can be started from Advanced Startup.