Whether you start from the top or the bottom has nothing to do with it,, it's a very rough job, and he's better leaving plastering to a plasterer. How good is the chippy too, if he recommended the chancer. Pay him for the materials he bought, pay him for the boarding, but tell him he'll get nothing for the plasterwork, because you really will need someone to sort out the mess he made, and that'll cost YOU money.