Free and Open Source Software

'install Linux' not really going to be an option in this case

Urbackup. It does automatic backups of both files and whole disk image of all computers in the network and it's set and forget. Once the server is set, you just need to install the client on every machine you need to backup, it will take all the settings automagically from the network.

Veeam is not FOSS, but there is a free version, it's user friendly, and has agents for both Windows and Linux.

Not free as in foss. But free as in beer.

You can use xpenology or just a synology disk station with active backup for business. It does quite well with windows and just runs in the background.

Before using that I use urbackup ,which is FOSS. It also worked quite well.

Duplicati runs om Windows AFAIK.

Urbackup has windows and linux backup agents and I believe is open source, but I don’t know if it has a windows server.

Veeam is pretty good, but not open source and the community version is limited to ten agents.

I've used Kopia before and liked it. It's cross platform and allows you to browse back through folders on a timeline like macOS Time Machine.

Not Foss but very nice to use: Veeam

It's free for home and commercial use but limited to one backup job.
You could install a backup server version of it in a VM (Veeam B&R) for (I believe) 15 backup jobs in total.

I use it at home and we sell and deploy it at work so my opinion is biased but I really like the product.

You'll need to think of "backup" as a different thing if you are looking at the free space. For instance, I can backup my data onto discs, but it costs buying discs. I can also make lots of copies of my images and videos automatically using SyncThing (which is open source), but it requires multiple computers to really be considered a "backup".

This doesn't meet your criteria because not FOSS, but hear me out...

I used synctoy for a while and it can sometimes be glitchy. I switched to aomei backupper [sic]. It's basically shareware with a few nags. It seems pretty solid.

Only thing that I think would be better would be to use robocopy with the mirror switch. For help getting the syntax down and scheduling it with task scheduler, gpt3 should have you covered.

Robocopy is already built into win7, 8, 10, and 11.

For foss, I looked at bakula, kopia, and duplicati. It's been a while since I looked into them so I honestly can't recall why I didn't switch to them. ¯⁠\⁠_⁠(⁠ツ⁠)⁠_⁠/⁠¯

What's your backup target (to where are these backups being sent?)?

Are you looking for actual Backup-strategy apps or just data sync as a form of backup (I wouldn't recommend this approach unless you can setup multiple destinations and some form of differencing).

Also, how will backups be validated? Will there be a monthly check for a while to verify files? Maybe a quarterly restore test until you trust it's working right? Who's going to do this?

Just some questions to consider. Good luck, backup is so much fun!

Free but not open-source, Microsoft's own SyncToy. Used it for years to do weekly backups to SD cards. You can set multiple source/target combos and run one or all with a button thereafter.

You can use borgbackup with WSL, borg is great.

Delta copy uses cygwin rsync under the hood. Its not fully foss but has the underpinnings

Is it necessary to be FOSS, or is free good enough? If cost is the only driver and the backup is local to a regular device, I can recommend SyncBack for simple backups on their free version. I bought Pro several years ago and there is no ongoing licensing fee. When they add certain services I would find useful, or as tech progresses, I'll upgrade but there's no ongoing cost and it runs reliably for me. It's also very windows-centric in terms of its design and UI, which makes it relatively easy to use. Granted I wouldn't give it to my mother to set up for herself, but it will run unattended once set up.

I use FreeFileSync: https://freefilesync.org

Lots of good recommendations here!

For user friendly software Veeam Endpoint, Synology Active Backup, and Duplicati would be my top 3. They all let you restore easily with a GUI that is easy to navigate, support email notifications natively for failures/warnings, and support VSS on windows without external scripts.

Kopia, Restic, and similar open source projects also work well on a technical level, but are about the farthest thing from user friendly as they require custom scripting to perform VSS snapshots before backup and send notifications after, and are not very easy to use for restoring files.

As far as I know Duplicati has a Windows version

Rsync on a scheduled task is dead simple. If your friend can stomach WSL (Windows subsystem for Linux) being on the machine you might try using that. The command is a one liner:

$rsync -r /path/to/source /path/to/backup

and can be automated with cron.