There is no problem with your client having GPL code if the code is not going to be published. As far as I know, GPL and GPL2 should be totally ok. The rights to the codebase can also be transferred but the GPL parts would retain the same license. So the new owner would have to publish it under GPL if he publishes the code and so on.

But proprietary code is the problem in the equation. They are not GPL and any code that gets bits of such proprietary code would be in violation of the copyright...