If a person negligently, or through carelessness--i.e. "accidentally," if that accident is the product of carelessness--causes damage to another's property, he or she could be held liable (financially responsible) for the damage. A determination that the fire was an accident, as opposed to deliberate, does not necessarily mean that there is no liability. Therefore, if the landlord feels your boyfriend was careless (e.g. that he shouldn't have had a butt bucket at all; or that he carelessly let the water level fall too low; or that he had tried to throw a butt in it, but missed and let it smolder elsewhere; etc.), the landlord may ask for him to cover the landlord's out-of-pocket cost (the deductible). If  your boyfriend disagrees, he can refuse to pay and let the landlord try to sue him and prove his case in court.