Whatever you choose, make sure you're familiar with it since you'll be the one that has to fix everything that's wrong. 🙂
I suggest not giving their user sudo rights and having your own user with sudo rights for installing apps, doing upgrades and so on.
It will be very useful to have SSH installed if you need to assist them remotely.
If you want to help remotely I also recommend Tailscale, it creates a "mesh VPN" private network where your PC and their laptop can see each other over an encrypted connection that can also break out of ISP NAT (no port forwards needed). Since it's encrypted it's ok to use simple unencrypted VNC to view their desktop to help when needed.
I can give some pointers if you have a home server and want them to be able to use web apps on it over Tailscale. One very useful example is Syncthing, which can sync files between a folder on the laptop and your server, where you can back it up further incrementally with Borg Backup or whatever you use. You can sync their entire home directory if you want or you can just have a ~/Sync dir where they put only what they want.
Last but not least, if you can swap the HDD consider putting in a SSD instead, the difference will be night and day.