As a rule, unless they're using a particularly unique piece of software OR running a complex set of PCs over a local intranet, then Win10 is a smashing OS and they'd be wise to take the free upgrade.
Don't get me wrong, I liked 7 and 8.1, but 10 really is an a different level. It's even made it possible to run some really old DOS stuff that refused to do anything on the older systems, so I'm all for it. But yes, you can permanently disable the update by installing third-party software that can block it.