nightcraw1er.488: Win10 cannot erase your gog account, the gog account is not on your machine, sure it might affect the data on your system, and you have a right to complain to windows, is this the windows forum... as for your gog account, foto the signin on the main website page, put your password in and voila, there is your gog account.
GOG has serious issues with the two-factor login authentication. I was traveling in January, when I wanted to take advantage of a sale. I haven't logged into GOG from my laptop previously, so I had to wait for the login code. Throughout the week I tied to log in, I never got the code promptly enough to log in. After I returned home, I went on my desktop and disabled the two-factor authentication, so that in the future I can log in from other devices. So, if he was logged into the site only from the one computer and had two-factor authentication turned on, there's a chance he really lost his access.
nightcraw1er.488: Maybe also pop over to amazon and get a back up external drive or two as well.
I did that as myself. My WinXP computer finally died two years ago, and I had to switch to Win 8.1. Took me another week to tweak everything to look like WinXP Classic again. Since then, I had to start using Win10 at work, and I'm very unhappy with it - even after all tweaking it acts too differently to be comfortable with it. As a result, I disabled all automatic updates to my Win 8.1 computer, so that it doesn't experience a forced switch. Which is why I am pretty good at backing up my system, and my restore disks are always ready :)