Plant Dying???

Hi ya peeps, My mate has a plant that he is growing (getting piccies later), but he thinks its dead.............He doesnt get alot of darkness in his place so decided to stick it in a cupboard (which my plants thrive on darkness) to see if this would help........since then he says the leaves have turned brown and are falling off now he bought some tomato feed from Wilko's which I have used on mine and my plants are fine. What do you think he has done wrong and is it fixable???

P.S These are indoor plants grown with no artifical light just Naturally grown