He's still a shareholder. It is considered extremely bad form to start a company that competes with a company that you are still a shareholder of unless you have not been fired with cause. The only other example I can think of is Jobs. So it is possible but I would assume that post Jobs contracts were put together in such a way that it would be very hard to compete with the company you got booted out of and that you are still a shareholder of. I've signed a couple of contracts over the years just as shareholders and those already mentioned that if I had competing interests the deal would be null and void and I'd be liable, and for good reasons too.