Not worth from such an employee to make a potential enemy, or break all contact with him, a man still can play service not only in human relationships but can help in the promotion of business. Plays favorite the human factor.

Even after retiring nobody can predetermine how will the employee work in the new company. There is absolutely no guarantee that he will stay there to work, for two reasons: he could promise the conditions that in fact are not complied with, a new employer may not like an employee. Then there is the opportunity to get a valuable asset back in service.

If the employee at the new place, things went uphill, you can always ask how he's doing. This is best done in an informal setting. Then the conversation he can share important information that can help to rebuild in time their work and benefit. Sounds like the old Russian proverb: "forewarned is forearmed".

Another alternative, which may happen in the future, cooperation of several companies into one large holding. Then such a worker could become an ally and partner. In the presence of normal human relationships, such a person would be much easier to create something completely new and to make the collaboration most fruitful.

Business is a predictable process, but even the most experienced analysts can't always predict future changes. So you need to be ready for anything, this could be a high rise and a very sick drop, but maintaining a friendship or just good relations in any case will play a role. A former employee who joined a new job, will always be able to recommend another employee, a professional in the business as my replacement.