Say what you will about the glories of Linux, but lets face it, it is not for the computer illiterate. You have to know what your doing if you want to game on a Linux machine. I have a friend of mine with an Alienware M17XR3 laptop computer and she for whatever reason put Linux on it. She has learned that Linux isn't the most gamer friendly and wants to SWITCH back to Windows 10. How do we make this happen?
From your description it appears she has a laptop on which she installed Linux. USING Linux, she can reduce the SIZE of one or more PARTITIONS and make room for Windows 10. Windows likes to have a primary partion.
She then needs to boot up Windows 10 install DVD and make a custom installation of Windows 10. It will ask for partition or free space.
Windows 10 should install the boot loader that will fit give her the OPTION of using either system.
 Geek it seems they want Linux gone so your advice may not apply...

See Here...

If she had Win10 already installed prior you are already registered for that PC with MS...