Comment by jeroenhd

For what it's worth, Windows 10 will sunset in two years. You can pay $30/year for security updates (or get updates for free by enabling Windows Backup or spending Microsoft Reward Points you earn by using Bing) and keep using Windows 10 until programs stop working, which will probably take a while with the current status.

My advice on dual booting Linux in general:

1. Resize the partition from Windows if you're going to install to the same disk

2. Do not use MBR mode to install anything. Windows updates will break your Grub bootloader, and Grub updates will break the Windows bootloader. Most PCs default to UEFI these days already, but it's always good to check

3. If at all possible, use the computer's UEFI OS dialog rather than chainloading Windows from the Linux bootloader. Kind of requires a user-friendly motherboard GUI, but if Bitlocker decides to turn itself on, you'll be struggling with recovery keys a lot less if you let both bootloaders just manage themselves.

4. If that's not possible or the motherboard OS selection sucks ass, and Bitlocker is enabled, write down the recovery key, boot through the Linux bootloader once (Windows will notice the boot process changed and prompt for a recovery key), and enter the recovery key. After a reboot, Bitlocker should unseal when booting from the Linux bootloader automatically, and now directly booting from the motherboard OS selection will require the recovery key.

5. If you're going to use a new disk, make sure to create a sufficiently large UEFI partition (>1GB, storage is cheap these days!) so bootloader/kernel updates don't break in the coming years.

6. Not really a dual booting issue, but if you're doing tech support for your relative, do yourself a favour and set up a tool like Timeshift (including automatic snapshots on updates, and if your configuration permits automatic integration into Grub). It'll offer system restore-like features, allowing you to revert the system to a known-good configuration in case an update breaks something. Timeshift itself works best on btrfs installs, but it'll also work in rsync mode on any file system, and other tools like it exist.