In the UEFI firmware settings, set the Windows drive to boot 2nd.
Save & exit.
to get into the missing Windows entry from grub connect a NON bootable USB stick.
Boot up & boot from the USB NON bootable stick.
(Depending on your hardware, you need to press a function key at boot to bring up a list of connected devices. Most hardware uses F8 or F10 or F12 to accomplish this)
Select the Non bootable USB stick.
The bios will then choose the 2nd boot drive getting you straight into Windows. Easy as.
Next boot without the USB stick connected you go straight to grub.