Microsoft's Windows Subsystem for Linux is coming to all Windows 10 users (archive):
You won't have to be a tester to try Windows 10's new, built-in Linux kernel in the near future. Microsoft has confirmed that Windows Subsystem for Linux 2 will be widely available when Windows 10 version 2004 arrives. You'll have to install it manually for a "few months" until an update adds automatic installs and updates, but that's a small price to pay if you want Linux and Windows to coexist in peace and harmony. It'll be easier to set up, at least -- the kernel will now be delivered through Windows Update instead of forcing you to install an entire Windows image.
Embrace, Extend... Excite!
Windows blog post.
Previously: Windows 10 Will Soon Ship with a Full, Open Source, GPLed Linux Kernel
So if you have the right version, I guess you can use a microSD card to facilitate an update.
He probably can't. Many cheap Atoms have platform drivers that weren't packaged in Windows that supply the SD card driver so the upgrade boot environment can't reach the extra storage.
In one particular Bay Trail device I couldn't even work the keyboard, touch screen, ethernet or bluetooth so I ended up doing the USB installation from the media creation tool on the one available USB port and a powered usb hub connected through a USB OTG on the power port chaining a keyboard, mouse, and a usb-ethernet adapter. I also weren't supplied drivers from the manufacturer and had to collect bits and pieces from similar lenovo, intel nuc and dell when the installation was done to get everything working...