This is probably not the better argument, although it is not altogether clear whether the courts will adjust the 'reasonable person' standard to allow for disability, incapacity or infirmity in particular defendants.
It seems likely that, even if a defendant's mental illness is attributed to the 'reasonable person', the risk of harm will be considered foreseeable as long as the defendant was not so incapacitated by their mental illness that they lacked any awareness of what they were doing.
It seems clear that a defendant will be liable if they had any advance warning of a sudden onset of their illness or infirmity.