Its an old laptop so the district has to be light weight. It's run mint mate before.
The main concern would be something that doesn't need active user maintenance for things like updates. Something I can get running and then leave with him. If I can have it autoupdate, or even a distro that can go without updates.
He's with a collection agency, so the "work" will be done via phone. He needs the laptop to access the accounts, presumably through email or company website. So nothing special as far as I know.
As for DeepinOS, yeah it's some Chinese distro I found out about. I forgot I tried it out and it just happens to be what is still installed on that laptop. It's pretty simple and user friendly. I thought some of the UI translations were funny though, like the mac-like UI is called "Fashion made" and the windows-like UI is "efficient-mode" haha XD
Let me tell you something about idiot-proof technology.
I've had this calculator for a couple of months. Typical office calc with a couple of extra functions, but not like those ultra-scientific calculators with graphing and alt-functions for every button... anyway, today I found out that the screen for this calculator can actually tilt upwards... literally after more than a month of use, I just looked at it from the side and noticed something like a hinge. Mind blown. Felt like an idiot.
But anyway, you've got a couple of good suggestions here, Debian has my vote for what you need. As for the 'lightweightness', Debian won't make any problems, but some desktop environments (like KDE) could, depending on how old the laptop is. The most stable desktop environments in the lightweight category are XFCE and LXDE (someone can correct me if I'm wrong).
I think either Cinnamon, or Mate present a pretty familiar desktop paradigm for someone new to Linux.
I don't think that you can make it unbreakable. He'll need either the root password, or sudo to do updates and with that, he can create all sorts of havoc. Perhaps you can create a regular user account for him, that has update privileges, but no other advanced capabilities?
EDIT: I expect that you are already aware that with Mint / Mate, you can configure it to show him only critical security updates. The other updates could wait until your periodic visits.
I have Solus / Mate running on an eleven year old Athlon2 machine and it runs great, thanks to the optimizations to this distro. Is shows all updates, but it groups them so that only mandatory security updates can be selected/installed, while the others can wait for another day.
All the kewl kids seem to be using Ubuntu's Mate distro these days. I haven't tried it personally, but it may be worth a look.
yes, though its not as up to date as the latest Ubuntu ditros are. My gf tried to use it because she liked the UI and the installer would crash because it didn't like the latest AMD GPU's (R9 390).
Yeah its some Chinese piece of garbage. It is based on Debian, but i think not really up to date. Allthough the Deepin desktop envoirement is also used on some other projects. Like the community spin of Manjaro Deepin etc. It has some fan base.
All good suggestions on distributions. OP mentioned may only need web browser so would suggest checking Peppermint and Lubuntu out too
Regardless of choice of distro, best option to make it "unbreakable" is install same distribution twice. If main installation has any problem, user only has to sort out down arrow in boot menu to load backup install and he can be back to checking email in minutes