It's not Microsofts fault, it's not the game developers fault either.
The software was designed prior to Windows 10. Whilst some support/updates would be the right thing to do, they are not obliged to either.
I'd say 'source' it elsewhere, you paid/own it - certainly no moral issues as far as I can see.
IMO they are releasing software for the Windows platform and should make an effort to maintain compatibility with future patches, service packs and updates whether to 7 or a future OS within reason - though at the same time 6+ years is starting to push the boundaries of what I'd expect in the way of continuous support even for commercial software (unless it was subscription paid).
What I haven't quite understood yet is why you don't just run it on W7, say via dual-boot?
If he has used his 7 key as an upgrade to 10 that would mean buying a new version of 7 to do that.