For all the reasons mentioned in “Making Sure the Applicant Starts” you must continue a supportive dialog with your applicant. You have a responsibility to him/her and the company. You have played an important role in a major change in this individual’s life! You are still the go-between him/her and the company. He/she can say things to you that can't be said to his/her new boss. The new boss can say things to you that can't be said comfortably to the new employee. You can intercept, interpret and clarify things that could lead to 'getting off on the wrong foot.'
Do your job, continue to communicate to applicant and client.