I don't think removing covers causes any issues and as such I doubt it voids the waranty - But I have no idea.
But putting them back on without wax potting certainly will, You can't just stick a slab of metal infront of a magnet without preperation :P ,
That being said, if he's the original buyer, I'm sure Tim will help him out, but it does seem a bit of a slap in the face to send back for a free service with the sole incentive of selling it - But leave the moral issues aside I guess.