If he's no longer a player he has no rights over his PC anyway.
With all due respect, S'mon, I'd have to disagree with you here- the GM creates the world in which the PC operates, but the player creates the PC.
Unless you're the kind of GM who actually takes posession of character sheets after each session, made players sign employee contracts or started the campaign entirely with his own pregens, that player is free to walk out
with his PC at any time.
AFAIK, the closest analog to the DM/Player relationship that's seen any kind of true contractual IP agreement would be those that shape "shared universe" fictions- Thieves' World, Wildcards, and so forth. In each of those (of which I'm aware), the characters were freely used by all the authors in the shared universe, but major character changes required clearance with the author who first created the character- creative control was retained by the originating author.
Of course, there is nothing I can think of that would prevent a creative GM from having the PC disappear mysteriously and replacing him with a capable NPC, or (as has been suggested numerous times above) duping the PC and running it as an evil twin/doppelganger/changeling, etc. The Player retains creative control over the true PC, after all, and the story marches on.
Heck- the return of the true PC could be just as dramatic as anything else in the campaign. Imagine the other PCs encountering their lost buddy and boisterously recounting how they uncovered his doppelganger...only to be met with a bewildered stare and "Excuse me, do I
know you?"
At any rate, all legal issues aside, respecting the wishes of the player who is leaving will probably minimize any hard feelings that may otherwise be engendered. That could mean a lot if and when that player returns.
It could even make the difference between "if" and "when."