My sentiment is that if he left the game, came back specifically to find someone hiding via spectate, then !zspawned to go to you, the ban was justified.

Joining mid-round and spectating someone then !zspawning to go to them is a very lame thing to do. Bannable? I'd certainly be pissed off it if happened to me, but I'd have to say not bannable unless compounded by other things. I know that if I joined a game mid-round and saw someone via spectate hiding I'd just act like I never saw them. Then again, I do that in several instances. For instance if the way I got to someone is through a virtually non-repeatable glitch I tend to just !ztele away or if I become a zombie in a group of people, same deal.

ZM maps are about surviving in any way you can; hiding is definitely one method. There are major drawbacks to hiding, too: you don't get kills and there's already a very high chance that you'll be found and die. I've gotten very good at tipping over vending machines for that very reason.