In this scenario, Stone is responsible for the accident. Because he hired the janitor. He should have hired a trained person for this job, or he should have given him proper training of his work. Also the janitor must have received the safety training when hired.
So if Martha fells down, Stone is liable for this accident. Because he could fire the janitor for his negligence but this type of accidents may occur in the future if he continue to hire unskilled and untrained people for the job. So stone is responsible for this accident.