Tenant rights for uninhabitable room?

One of the rooms in our flat has really bad mould, damp, leaking roof and is need of repair. Agency and landlord is aware of this and have proceeded to knock a hole in the wall for ventilation..covered by a grill that can't be closed. The dude in the room has now had enough and is moving out. The room has made him ill (nose bleeds, coughs, wart growths) not mentioning the freezing conditions from the open grill. He's forfeiting his deposit, fair enough but we know the room is a mess..what are our rights as tenants? we are in a joint tenancy but no-one can live in that room what do we do?