Windows 10:
This is NOT irrelevant. It's being offered as a FREE upgrade to Win7 or 8. Even the PreRelease version right now could be applied and it will update automatically to the full version later with no reinstallation. I believe after release Microsoft will offer an "upgrade" via Microsoft Updates though I'd recommend burning a DVD and doing a clean install.
Can he just "swap" the motherboard and keep everything as is?
Maybe. Seriously, it doesn't hurt to try (after backing up some data). If it works fine then you're done then maybe do the Windows 10 upgrade later.
Have I done this myself?
Yes, several times. Years ago with XP it was almost guaranteed to fail. I've had mostly success in recent years. Usually it's almost guaranteed to work if the same chips are used.
Would I recommend reinstalling Windows?
Yes, in general. However I know a lot of people who aren't very great with computers who will end up losing a lot of data despite being told to backup so simply swapping and seeing if it works is a pretty good option.
Again though, no matter what plan out your strategy.
Other:
OEM needs new key for new motherboard?
I don't think so. Again, it's considered a repair build if you swap the motherboard. If you swapped to a different SOCKET (i.e. upgrading motherboard, CPU) that's a different story.