If simply educating them on what not to do (clicking on 'free' stuff, downloading without discretion) won't work, I'd suggest switching out the OS to something

If simply educating them on what not to do (clicking on 'free' stuff, downloading without discretion) won't work, I'd suggest switching out the OS to something a little less targetted by malware. I recently got my mom a chromebook, which she loves. You say your dad uses lightroom though, so that might not work for your use case.