Use Linux. If he only uses his computer for browsing the Internet and editing photos linux mint (mate or cinnamon) will do fine. Show him how to use darktable instead of lightroom and that's it. No more problems.
I have (had?) one client that every week or the other would come back for a Windows reinstall. Since he only browsed the internet and did not use any particular software, I installed Linux Mint and he never came back! I lost that steady 'revenue' though.