Agreed, use Virtualbox, get the windows 10/11 iso from microsoft's own site, install windows in a VM just like you would another PC. I've been doing this for ~20 years, as long as I've used linux as a full-time system, and the only way to run windoze.
This also comes with the advantage of being able to leverage Snapshots. As soon as your clean windoze vm is installed, registered all shiny and unmolested, create a Snapshot of your image. Should your Dad get infected in any way, or even just suspects it, before any upgrade, anything dubious, just restore to that Snapshot, and good as new again to a perfect state.
I always get keys from
gkeys24.com, win10 is like $7 bucks for a legit key, win11 a bit more (but ew). I can forward you a latest ad, they tend to hide windoze licenses from the main site to avoid microsoft's lawyers (I assume).
-mb