There's plenty of legal precedent for this kind of thing in Non-Compete and Non-Disclosure laws, especially in creative industries. I don't have a concrete example (sorry), but I know that places like Disney have them. A Disney employee can't sit around thinking up scripts and selling them on his own- Disney will claim that they own whatever creative output he has while working for them.
The question here is how common it is in twinn's industry and whether it seems ethical and worth putting up with. For a programmer to be barred from freelancing, consulting, or side-projects AT ALL is pretty extreme. But it's probably easier for his company to leave that in place instead of trying to define what is or is not competing with their product line.
The question here is how common it is in twinn's industry and whether it seems ethical and worth putting up with. For a programmer to be barred from freelancing, consulting, or side-projects AT ALL is pretty extreme. But it's probably easier for his company to leave that in place instead of trying to define what is or is not competing with their product line.