They only have to buy his vested equity. His unvested equity, for all practical purposes, reverts back to the company. In standard 4/1 vesting, you get 1/4 of your equity on your first anniversary, and then equal-sized chunks every month thereafter for 4 years. Prior to that first anniversary, the idea of 4/1 is that you get nothing; the whole idea is to avoid allocating equity to people who don't last a whole year.