Based upon your wording, I am assuming your father is not particularly tech savvy, if this is the case first and foremost you should be picking a distro that is maintained by a large group of trustworthy developers, this removes the niche distros from the running. Secondly, since he isn't going to want to learn the terminal, you should be picking a distro that installs programs with a GUI package manager or flatpak manager, this removes the likes of arch, gentoo, & open suse tumbleweed. Thirdly, you will want a distro that is based on one you understand well enough to run tech support, I don't know which that is for you, if it is Debian based stick with mint, fedora based go with fedora workstation or fedora KDE, if it is opensuse I don't have any recommendations sorry.

After you select the distro you need to educate your dad that he should only be getting new programs through the package manager, and I would either tell him the inherit insecurity of some flatpaks or remove flathub from your mirror list unless there is something he really needs in which case you need to do your research.

In general security on Linux is a lot more active for IT than it is for Windows, but for the general user if they can get by using a well known distro's repos you shouldn't have any security issues.

If you are overly worried you could add apparmor to the system to isolate the system from programs or pick an immutable distro like bazzite, but in general the immutables are smaller teams which is why I don't prefer them.