There should always be a backup plan if this situation every happens and I don’t believe that someone can leave without month or two week notice. But even though this happens I would have contact info from former worker and ask for all the notes, tests, plane,etc. preformed so for and if he was waiting for results from someone, so I would update new worker. I would prefer to have two workers so they can tackle the task faster because if I only have one worker trying to learn everything and perform task will be time consuming and time is precious. To keep on schedule I would have two workers working and later reduce to one worker. I would also work with them and assist so I can give a hand because as project manager I is his duty to communicate and make sure everyone completes their task.