Ms. Seema,
To add,
Does your company have a high attrition rate, a hire-and-fire policy, victimization of managers, low morale, bad managerial and HR practices, etc.? It is a time for corporate introspection - of course, you could be personally helpless unless this is an isolated case, and you have a professional, progressive, introspective culture where management is open to taking corrective cultural initiatives in today's age of enlightened HRD culture.
Maybe the next employer has assured him of all support while joining the organization, and he has taken a calculated risk. Some companies do not relieve a person timely or settle his accounts promptly, even though a thorough resignation has been submitted, finding excuses to extend his stay to complete assignments, etc., as a matter of right over the employees. Did he have any genuine grievances, or were his relations with the top management or his boss impaired for any known reason? This may have a demoralizing impact on the managers and employees down the line.
Let some very senior company official have an out-of-the-campus transparent discussion with him at a hotel/restaurant, assuaging his feelings. Since he has already joined elsewhere, help can be requested on critical information or pending matters by visiting the office in the evenings or on holidays and keeping the communication lines open and friendly. He could be requested to hand over the files/documents he has to an authorized person nominated by the company, simultaneously handing over the check for his dues after deducting the notice pay due to the company as a matter of policy. Invite him for a farewell party one evening where the misunderstandings leading to the situation could be clarified, assuring other managers of a transparent, enlightened management able to handle the situation coolly, preventing further damage of morale in the organization. Hope all this is possible - could enhance the corporate image and the employer brand value to be repaired and encashed, at least a theoretical possibility if one would like to learn to handle emotions rationally and act wisely enough. Hope this makes at least an interesting read for ourselves.
In one company, a very senior CEO joined a competitor, attending his exhibition even before he had officially left the company, which the MD did not like. His dues were cleared, invited to the office and handed over - over a bouquet of flowers and some ice cream in the MD's office, avoiding a group farewell to prevent any further embarrassments that could crop up. Thus, things could be handled amicably to the extent possible.
Earlier, it was labor/industrial relations handling matters legally through legal documentation and legal games, now it is human/employee relations making the workplace a better democratic place with a proactive people-oriented culture, leaving no room for such incidents as above, a senior person leaving without intimation, which does not speak well for the company culture too with a need to handle the situation with soft options possible, reducing the cultural damage - deploying the much talked about soft skills in practice, unless the facts warrant a legal punitive action beyond doubt.
Hope this helps academically at least.
Regards and best wishes,
Kshantaram