I am not an expert on Windows Activation, but I think that this depends upon what kind of license you have. I know Technet/MSDN install keys allow you to use the same key on up to X different machine "fingerprints" depending upon what type of subscription you have, so in this case I think he'd be OK. It should just look like 2 of X activations on different machines.

If you tried doing the same with an off-the-shelf Windows 7 upgrade install or an OEM install, I'm pretty sure you'd run into problems, though, unless the two machines were identical from the point of view of Windows Activation.

No it's absolutely fine. Once activated, it's pretty much 100% ok if you move it around.

It used to be an issue in Vista days but no longer - they have relaxed most requirements.

I think it's mostly OEM editions that are restrictive. "Standard" licenses give you more freedom.