For configuration you can help them with TeamView or something similar, usually parents don't go and buy printers or video cars without asking you if you are the one that is more technical.
But if you already ton't help them cleaning Windows and instead sending them to a store then sure let them continuing use Windows.
I am a Linux user and Free software supporter but I would never try to switch someone that is happy with his system, I am lazy and I hate taking responsibility for the system. (as an example a friend of mine fixed someone PC by reinstalling Windows but the sound would not work because of missing drivers (he did not had internet do find drivers) and he was suspecting of stealing the sound card or breaking the sound somehow... so yeah if you don't care send them to a store)
Exactly because I care to have a life outside work, and don't want them to have to depend on someone located 3000 km away, that I rather have them going to the local PC store.
Actually, yes my parents do buy stuff on their own, or by talking with friends of theirs, to whom Linux is that strange thing that some "university guys" use.
And to make it clear, it isn't a random PC store that does dirty tricks, just like many random car repair shops happen to do, rather something proper.
Additionally any dude at the local PC store can help with configuration issues during warranty.
Or when their network goes tits up and the calldesk from the network provider only has support scripts for Windows or macOS.