Try using a USB boot stick to boot into Linux and just save whatever you need from the machine and then reformat it. Since this way the machine won’t automatically connect to Wi-Fi and potentially cause more issues.
That’s the quickest, simplest way without needing to try to diagnose and dig into the system to see what is affected and trying to fix it.
Also what is the computer doing when it boots up? There’s not really enough information being given to be able to provide any other advice.
Hopping on a live USB to recover files is your safest option.
It will also give you an opportunity to scan files (with something like clam av) while running from a system other than Windows, so you’re less likely to encounter any further infections. Not that Linux can’t be infected, it’s just much less likely and you’d be running from a flash drive and off network anyway so it’s about as safe as you can get.
You would need to connect the live USB to the Internet to install clam av on the USB stick or something similar, but that can be done while using a separate machine before actually plugging into the affected machine.
I can’t really offer any advice on using any software for scanning as I keep personal things on separate drives segregated from the network so if something ever did happen I’d just wipe and start over.
May be a good idea to take though. Get him a USB drive that he can store files on and disconnect when he doesn’t need it.
Just some thoughts from someone that works in desktop support and has been tinkering for a little over 20 years.
Good luck!